• 3.1 Applicability and Acceptability of Individual Traffic Calming Measures
• 3.2 Cost of Individual Traffic Calming Measures
• 3.3 Temporary Versus Permanent Installation
• 3.4 Lateral Shift
• 3.5 Chicane
• 3.6 Realigned Intersection
• 3.7 Traffic Circle (Not Roundabout)
• 3.8 Small Modern Roundabout and Mini-Roundabout (Not Traffic Circle)

For each of the traffic calming measures covered in this ePrimer, the following four sections are provided:

1. A description of the measure and its general purpose,
2. An overview of the setting where each measure is appropriate,
3. A summary that highlights the key effects and issues associated with the measure that are essential to address, and
4. A sampling of additional key design considerations for the measure.

[The descriptions of various traffic calming measures on the following pages mention the need for appropriate signs and pavement markings. The MUTCD, supplemented by state and local practice, should be the source for specific traffic control device guidance.]

3.1 Applicability and Acceptability of Individual Traffic Calming Measures

Table 3.1 presents a simplified summary of the potential applicability of each individual traffic calming measure and the likelihood of its acceptability for a particular setting. The screening is presented in terms of the location (intersection or roadway segment), the roadway functional classification, and other attributes of the roadway function (emergency services access requirements, presence of a transit route). It is critical to remember that the applicability of a particular traffic calming measure has as much to do with the problem to be addressed as the physical setting elements listed in the table.

A more comprehensive assessment of measure applicability is presented in a distinct table for each individual measure. Table 3.1 can be used as an initial screening tool to identify whether a particular traffic calming measure has a likely fatal flaw in terms of its overall applicability and acceptability. For example, the table illustrates that neither a speed hump nor traffic circle is an appropriate measure along a thoroughfare or major street.

Table 3.1. Likelihood of Acceptability of Traffic Calming Measure

Traffic Calming Measure	Segment or Intersection	Functional Classification			Street Function
		Thoroughfare or Major	Collector or Residential Collector	Local or Local Residential	Emergency Access	Transit Route
Horizontal Deflection
Lateral Shift	Segment	3	5	5	5	5
Chicane	Segment	1	5	5	3	3
Realigned Intersection	Intersection	1	5	5	5	5
Traffic Circle	Intersection	1	3	5	3	3
Small Modern & Mini-Roundabout	Intersection	3	3	5	5	5
Roundabout	Intersection	5	3	1	5	5
Vertical Deflection
Speed Hump	Segment	1	5	5	1	3
Speed Cushion	Segment	1	5	5	5	5
Speed Table	Segment	3	5	5	1	3
Offset Speed Table	Segment	3	5	5	5	3
Raised Crosswalk	Both	3	5	5	1	3
Raised Intersection	Intersection	3	5	5	3	3
Street Width Reduction
Corner Extension	Intersection	5	5	5	5	5
Choker	Segment	5	5	5	5	5
Median Island	Both	5	5	5	5	5
On-Street Parking	Segment	5	5	5	5	5
Road Diet	Both	5	5	3	5	5
Routing Restriction
Diagonal Diverter	Intersection	1	3	3	1	3
Full Closure	Both	1	3	3	1	1
Half Closure	Intersection	1	5	5	3	3
Median Barrier	Intersection	3	5	5	1	3
Forced Turn Island	Intersection	3	5	5	3	3

Legend:

5 – traffic calming measure may be appropriate
3 – caution; traffic calming measure could be inappropriate
1 – traffic calming measure is likely inappropriate

Note: Refer to individual traffic calming measure section for a complete description of the appropriate application of each measure.

3.2 Cost of Individual Traffic Calming Measures

Table 3.2 presents a range of cost estimates for each individual traffic calming measure. The cost of a measure is an important consideration in its evaluation and, ultimately, selection. The wide variance in the cost estimate for each measure is due to the following five key factors:

• Size – the area covered by a traffic calming measure can significantly influence the cost (for example, a forced turn island at a local residential street intersection is likely to be smaller than one provided at a collector/arterial intersection);
• Scale – the overall project scale and number of measures constructed has a significant impact on the cost of a project (for example, the unit cost per speed hump for a single installation can be significantly more than for a series of speed humps);
• Landscaping – the extent and type of landscaping (and the cost of providing the appropriate environment in which to flourish) can have a wide cost range;
• Drainage – the addition of a traffic calming measure may influence the drainage of the roadway and improvements would be required to maintain proper roadway drainage; and
• Utility access points – the relocation or redesign of access to drains, valves, etc. can represent a significant cost

The Pedestrian and Bicycle Information Center provides a link to a document that also provides cost estimates for a variety of measures that encompass pedestrian and bicycle infrastructure improvements, including some traffic calming measures (www.pedbikeinfo.org/planning/facilities.cfm).

Table 3.2. Approximate Implementation Cost for a Traffic Calming Measure

Traffic Calming Measure	Typical Cost for Implementation1			Comments
	Low (<$6k)	Medium ($6k-$15k)	High (>$15k)
Horizontal Deflection
Lateral Shift		Medium
Chicane		Medium		Between $8,000 and $10,000 for typical small chicane with simple design; as much as $25,000 for replacement of existing curbing or modifying drainage structures
Realigned Intersection		Medium
Traffic Circle		Medium		Typical unit cost around $15,000 with common range between $10,000 and $25,0002
Mini-Roundabout		Medium	High	Typical range between $15,000 and $60,000
Roundabout			High	Typical range between $150,000 and $2 million
Vertical Deflection
Speed Hump	Low			Typical unit cost ranges between $2,000 and $4,000; costs ranging between $1,000 and $8,000 have been reported3
Speed Cushion	Low			Typical cost for set of rubber cushions ranges between $3,000 and $4,000; for asphalt set, range between $2,500 and $6,0004
Speed Table		Medium		Requires more material than speed hump; typical unit cost ranges between $2,500 and $8,000
Offset Speed Table		Medium
Raised Crosswalk		Medium		Typically requires more material than a speed hump; cost ranges between $4,000 and $8,000
Raised Intersection			High	Wide range for typical cost – between $15,000 and $60,000 (and higher depending on width of intersecting roads and drainage requirements)
Street Width Reduction
Corner Extension		Medium	High	If drainage is not an issue, typical cost for four corner extensions ranges between $8,000 and $12,000; if drainage alteration is required, cost can increase to $40,000
Choker		Medium	High	Typical cost ranges between $10,000 and $25,000, depending on size of choker and drainage considerations
Median Island		Medium	High	Typical cost can range between $15,000 and $55,000; cost is direct function of length and width of median island5
On-Street Parking	Low
Road Diet	Low			Requires pavement markings, signs, and potential reconfiguration or adjustment of signals at intersections
Routing Restriction
Diagonal Diverter	Low	Medium		Typical cost for single diverter with limited drainage modifications is around $6,000; costs can vary widely based on size, drainage, materials, and landscaping
Full Closure		Medium	High	Simple closure can cost less than $10,000; complex closure with drainage modifications can cost as much as $100,000
Half Closure	Low	Medium	High	Cost can range from $3,000 for asphalt, pre-cast curb bulb with no drainage modifications to $40,000 for measure fully integrated into streetscape with poured-in-place concrete corner extensions, landscaping and drainage modifications
Median Barrier & Forced Turn Island	Low	Medium	High	Typical cost can range between $1,500 and $20,000, depending on length and width of barrier, construction materials, and landscaping6

¹ Includes costs for design, materials, and construction; does not include right-of-way costs

² Source: Seattle WA experience

³ Source: ITE Guidelines for the Design and Application of Speed Humps

³ Source: ITE Journal article "New Traffic Calming Device of Choice"

⁵ Source: Costs for Pedestrian and Bicyclist Infrastructure Improvements, Pedestrian and Bicycle Information Center (PBIC), Robert Wood Johnson Foundation's Active Living Research Program, Federal Highway Administration

⁶ Source: Costs for Pedestrian and Bicyclist Infrastructure Improvements

3.3 Temporary Versus Permanent Installation

It may be appropriate to install a temporary version of a traffic calming measure under certain circumstances. Examples include:

• When there is a need to verify that the location, configuration, and geometry of a traffic calming measure will produce the desired effect (e.g., vehicle speed change, motorist compliance, vehicle maneuverability), before investment in a permanent feature
• When there are insufficient funds available for permanent construction
• When there is a desire to gauge community reaction to, or opinion of, the measure before investment in a permanent feature; or
• A short-term initiative is needed to provide traffic calming on a local street during a major traffic generating event or nearby construction on the highway system.

Typical materials used in a temporary traffic calming feature include delineators, precast concrete curbing, removable rubber speed humps, precast or wood planters, rolled asphalt, pavement markings and signage.

The potential effectiveness of a traffic calming measure can likely be affected by the quality of its design and implementation. Therefore, the limitations introduced in a temporary installation may likewise limit the effectiveness of the measure.

3.4 Lateral Shift

Description and General Purpose

A lateral shift is a realignment of an otherwise straight street that causes travel lanes to shift in one direction. The primary purpose of a lateral shift is to reduce motor vehicle speed along the street. A typical lateral shift separates opposing traffic through the shift with the aid of a median island. Without the island, a motorist could cross the centerline in order to drive the straightest path possible, thereby reducing the speed reduction effectiveness of the lateral shift. In addition, a median island reduces the likelihood a motorist will veer into the path of opposing traffic, further improving the safety of the roadway for motorists. The schematic in Figure 3.4.1 illustrates the lateral shift concept. Figure 3.4.2 shows a lateral shift in conjunction with on-street parking.

[A chicane is a variation of a lateral shift and is treated as a separate type of measure in this ePrimer; a chicane shifts alignment more than once.]

Figure 3.4.1. Lateral Shift Schematic
(Source: Delaware Department of Transportation)

Figure 3.4.2. Lateral Shift with On-Street Parking
(Source: Google Street View)

APPROPRIATE APPLICATION

Appropriate Application – Lateral Shift
Type of Street	With the proper degree of horizontal curvature, can be an appropriate traffic calming measure for a local road, a collector road or even an arterial roadway (or major thoroughfare), where high traffic volumes and high-posted speeds preclude many of the other traffic calming measures Can be appropriate in both an urban and suburban setting
Intersection or Roadway Segment	Applicable only midblock (see photograph in Figure 3.4.3)
Roadway Cross-Section	Appropriate for use on a single-lane, one-way street and on a two-lane, two-way street Can be installed on a roadway with either an open or urban cross-section (i.e., curb and gutter) Can be applied on a cross-section both with and without a bicycle facility
Speed Limit	Can be appropriate across a range of speed limits, provided the lateral shift has an adequate taper and an adequate shy distance is provided between the travel lane and the lateral shift curb In practice, maximum speed limit is typically 35 mph
Vehicle Traffic Volume	Can be appropriate at all levels of traffic volume
Emergency Route	Can be appropriate along a primary emergency vehicle route or on a street that provides access to a hospital or emergency medical services A low narrow median can be straddled by a fire truck if needed
Transit Route	Can be appropriate along a bus transit route
Access Route	Can be appropriate along a primary access route to a commercial or industrial site
Grade	Can be installed on a crest vertical curve only if there is adequate stopping sight distance or warning signs are provided Maximum acceptable grade for a street with a lateral shift should be established based on local standards and experience; as an example, Delaware recommends a maximum grade of 6 percent

Figure 3.4.3. Midblock Lateral Shift
(Source: Google Street View)

EFFECTS AND ISSUES

Effects and Issues – Lateral Shift
Vehicle Speed	Can slow traffic by encouraging a motorist to moderate vehicle speed through the horizontal deflection; amount of speed reduction (or the final speed) depends on the length of the alignment shift, as well as the volume and distribution of traffic Less effective in reducing vehicle speed (1) when the volume of traffic is significantly higher in one direction than the other or (2) when volumes are so low that the likelihood of a motorist encountering an opposing motorist within the lateral shift zone is low Expected speed reduction through a lateral shift is typically less than that observed through a chicane
Vehicle Volume	Amount of traffic diversion depends on the amount of speed reduction, the increased travel time for non-local traffic and the availability of a quicker, alternative route As a single installation, there is little traffic diversion from the street
Pedestrian Safety and Mobility	Can be a location for a crosswalk
Bicyclist Safety and Mobility	On a street with a bicycle lane or substantial bicycle traffic, should either (1) provide a bypass lane for a bicyclist, separated from the main travel lane by a curb extension or (2) provide shared lane markings (sometimes known as a sharrow) and "bike may use full lane" signage
Motorist Safety and Mobility	Likely to have minimal effect on motorist mobility and safety Minimal impact on motorist comfort
Emergency Vehicle Safety and Mobility	Retains sufficient width to allow for the continued flow of emergency vehicles Refer to Module 5 for additional discussion
Large Vehicle Safety and Mobility	Retains sufficient width to allow for the continued flow of large vehicles like combination trucks Refer to Module 5 for additional discussion
Accessibility of Adjacent Property	May require removal of some on-street parking and may, therefore, slightly reduce the accessibility of adjacent property
Environment	Physical features can also be used as a landscaping opportunity (as shown in Figure 3.4.4)
Design Issues	Attention needed to avoid need to relocate drainage features (catch basins, concrete channels, valley gutters, inlets, and trench drains) Should not require relocation of above- and below-ground utilities

Figure 3.4.4. Lateral Shift Downstream of Signalized Intersection
(Source: Ian Lockwood)

ADDITIONAL DESIGN CONSIDERATIONS

A typical lateral shift separates opposing traffic with a median island (as illustrated in the sample design in Figure 3.4.5). Without a median island, a motorist can cross the centerline to negate the designed horizontal deflection and maintain a higher travel speed.

The expected level of speed reduction is a function of the amount of the lateral shift and the angle of deflection. A lateral shift of at least one-lane width and an angle of deflection of at least 45 degrees is a common target.

A lateral shift can be created by means of either a curb extension or an edge island. A curb extension offers a better opportunity for aesthetic enhancement through landscaping. But an edge island can leave an existing drainage channel open and tends to be less costly to construct.

Edge line tapers should conform to the MUTCD. The curb extension or edge island should have 45 degree tapers to reinforce the edge lines.

A curb extension or edge island that forms the lateral shift should have a vertical element (e.g., signs, landscaping), a reflector or some measure to draw attention to it.

Either a barrier or mountable curb can be used on an island that forms a lateral shift. The use of a mountable curb is more forgiving to motorists and is acceptable because the island is not expected to serve as a pedestrian refuge.

For a low speed street, the mountable curb may be placed at the edge of the travel lane rather than offset by 1 foot or more as is required for a barrier curb.

A midblock location near a streetlight is preferred.

Figure 3.4.5. Sample Design for Lateral Shift
(Source: Delaware Department of Transportation)

3.5 Chicane

DESCRIPTION AND GENERAL PURPOSE

A chicane is a series of alternating curves or lane shifts that are located in a position to force a motorist to steer back and forth out of a straight travel path. The curvilinear path is intended to reduce the speed at which a motorist is comfortable travelling through the feature. The lower speed could in turn result in a traffic volume reduction.

[A chicane is also known as a deviation, serpentine, reversing curve, or twist.]

[A lateral shift is a variation of the chicane and is treated as a separate traffic calming measure in this ePrimer. A lateral shift involves only a single shift in the travelway alignment.]

The chicane curves can be created with a curb extension that alternates from one side of the street to the other.

A chicane-like effect can also be achieved by alternating on-street parking from one side of the street to the other. Parking can create the chicane effect as long there is sufficient demand so that a majority of the on-street spaces are occupied during the times when vehicle speed is perceived to be a safety issue for pedestrians. Parallel parking, angled parking, or a combination may be used. Or it can also include landscaped curb extensions to beautify the street, screen the parked vehicles, and create protected parking bays.

[Three field studies of seven chicanes measured reductions between 3 and 9 mph for 85^th percentile speeds (Source: FHWA, Engineering Speed Management Countermeasures: A Desktop Reference of Potential Effectiveness in Reducing Speed, July 2014) http://www.safety.fhwa.dot.gov/speedmgt/ref_mats/eng_count/2014/reducing_speed.cfm]

The schematic in Figure 3.5.1 illustrates the chicane concept. Figure 3.5.2 shows an example of a chicane.

Streetfilm created a "Chicane Animated Traffic Calming" stop-animation film that demonstrates a chicane (Source: Streetfilms)

Figure 3.5.1. Chicane Schematic
(Source: Delaware Department of Transportation)

Figure 3.5.2. Chicane with Median
(Source: Scott Wainwright)

APPROPRIATE APPLICATION

Appropriate Application – Chicane
Type of Street	With the proper degree of horizontal curvature, can be appropriate for a local road or low-volume collector Can be appropriate in both an urban and suburban setting
Intersection or Roadway Segment	Applicable midblock or the entire block if the block length is short (see Figure 3.5.3 for an example)
Roadway Cross-Section	Can be used on a one-lane, one-way and two-lane, two-way road Can be installed with either an open or urban cross-section (i.e., curb and gutter) Can be applied both with and without a bicycle facility
Speed Limit	Can be appropriate if the speed limit is typically 35 MPH or less; in Delaware the recommended speed limit maximum is 25 mph
Vehicle Traffic Volume	Can be appropriate if traffic volume is relatively low; Pennsylvania sets a maximum of 3,500 vehicles per day Most effective in reducing speeds if traffic volumes are relatively balanced in each direction
Emergency Route	Can be appropriate along a primary emergency vehicle route or on a street that provides access to hospital/emergency medical services, provided traffic volumes are low enough to allow an emergency vehicle to straddle the street centerline
Transit Route	Can be appropriate along a bus transit route7
Access Route	Not typically appropriate along a primary access route to a commercial or industrial site
Grade	Can be installed on a crest vertical curve only if there is adequate stopping sight distance or warning signs are provided Maximum acceptable grade should be based on local standards and experience; examples include Pennsylvania with maximum grade of 8 percent and Delaware with a 6 percent maximum

⁷ As a transit vehicle travels through a chicane, the vehicle motion can decrease passenger comfort or contribute to slips or falls for standing transit passengers. Loose personal articles can also be dislodged within a transit vehicle.

Figure 3.5.3. Chicane between Intersections
(Source: Kristen Langley)

EFFECTS AND ISSUES

Effects and Issues – Chicane
Vehicle Speed	Can slow traffic by encouraging a motorist to moderate vehicle speed through a series of horizontal deflections; amount of speed reduction (or the final speed) depends on the length of the alignment shift, as well as the volume and distribution of traffic Less effective in reducing vehicle speed (1) when the volume of traffic is significantly higher in one direction than the other or (2) when volumes are so low that the likelihood of a motorist encountering an opposing motorist within the chicane zone is low May still permit speeding by motorists who cut a straight path across the center line; placement of a median island to channel traffic may be necessary
Vehicle Volume	As a single installation, there is little traffic diversion from the street
Pedestrian Safety and Mobility	Typically, not a preferred location for a crosswalk because motorist attention should be focused on the horizontal deflection
Bicyclist Safety and Mobility	Can narrow the travelway and force a bicyclist and motor vehicle to share a travel lane or provide shared lane markings (sometimes known as a sharrow) within the chicane, with appropriate signage
Motorist Safety and Mobility	Likely to have minimal effect on motorist mobility and safety Minimal impact on motorist comfort Aggressive motorist may view a chicane as an "obstacle course" with sharp cornering, braking and acceleration
Emergency Vehicle Safety and Mobility	Should retain sufficient width to allow for the continued easy flow of emergency vehicles; should have little effect on emergency response times Refer to Module 5 for additional discussion
Large Vehicle Safety and Mobility	Retains sufficient width to allow for the continued easy flow of large vehicles Refer to Module 5 for additional discussion
Accessibility of Adjacent Property	May require removal of some on-street parking within the chicane and may, therefore, reduce the accessibility of adjacent property Placement needs to consider the accessibility of driveways
Environment	Opportunity for landscaping Attention needs to be given to appearance to counter potential for visual clutter
Design Issues	Attention needed to avoid need to relocate drainage features such as catch basins, concrete channels, valley gutters, inlets, and trench drains (see Figure 3.5.4) Should not require relocation of above- and below-ground utilities

Figure 3.5.4. Chicane Designed to Retain Drainage Features
(Source: Google Street View)

ADDITIONAL DESIGN CONSIDERATIONS

The typical chicane separates opposing traffic by means of double solid yellow lines with recessed pavement markers. Even this may not be enough to discourage some motorists from cutting across the centerline to minimize deflection. To discourage this behavior, a raised median may be installed. The median may be narrow and mountable without landscaping. An alternative, if right-of-way permits, is to provide a wider and landscaped median.

A chicane can be created by means of either curb extensions or edge islands (an example application is shown in Figure 3.5.5). An edge island can leave an existing drainage channel open and tends to be less costly to construct than a curb extension. The typical chicane uses trapezoidal islands because experience has demonstrated the shape is more effective in reducing speeds than a semi-circular shape.

A curb extension or edge island that forms a chicane should have vertical elements (e.g., signs, landscaping) to draw attention to it.

The edge line taper should conform to the MUTCD taper formula. A curb extension or edge island with a 45 degree taper tends to reinforce the edge lines.

A mountable curb should be used on a curb extension or edge island that forms a chicane. A mountable curb is more forgiving than a barrier curb for a motorist that traverses the chicane and the edge island or curb extension that form the chicane is not an appropriate location for a pedestrian crossing (and therefore should not serve as a pedestrian refuge). For a low-speed roadway, a mountable curb may be placed at the edge of the travel lane rather than offset by 1 foot or more as with barrier curbs.

A sample chicane design is illustrated in Figure 3.5.6.

Figure 3.5.5. Chicane in Residential Area
(Source: Scott Wainwright)

Figure 3.5.6. Sample Design for Chicane
(Source: Delaware Department of Transportation)

3.6 Realigned Intersection

DESCRIPTION AND GENERAL PURPOSE

For the purpose of traffic calming, a realigned intersection is the reconfiguration of an intersection with perpendicular angles to have skewed approaches or travel paths through the intersection (as illustrated in the Figure 3.6.1 schematic). The expectation is that these physical features will remove or discourage fast vehicle movements through the intersection.

The most common application is the conversion of a T-intersection with straight approaches into curving streets meeting at right angles. The result is the removal of all straight paths through the intersection.

[A realigned intersection is sometimes called a modified intersection]

Figure 3.6.1. Realigned Intersection Schematic
(Source: Delaware Department of Transportation)

APPROPRIATE APPLICATION

Appropriate Application – Realigned Intersection
Type of Street	Appropriate for collector, local, and subdivision streets (see Figure 3.6.2) Can be appropriate in both an urban and suburban setting
Intersection or Roadway Segment	Typically applicable only at a T-intersection
Roadway Cross-Section	Can be used on both one-way and two-way streets Most commonly installed on a roadway with an urban cross-section (i.e., curb and gutter); could be acceptable for an intersection with shoulders only Can be applied both with and without a bicycle facility Can be applied on a roadway with or without on-street parking
Speed Limit	Should be in line with the nature of the street network; 25 mph speed limit is the most common maximum
Vehicle Traffic Volume	Not typically a direct consideration in determination of applicability
Emergency Route	Can be appropriate along a primary emergency vehicle route or street that provides access to a hospital or emergency medical services if appropriate turning radii can be provided
Transit Route	May be appropriate along a bus transit route if adequate turning radii can be provided
Access Route	Typically not appropriate along primary access route to a commercial or industrial site if adequate turning radii are not provided
Grade	Can be installed on a crest vertical curve only if there is adequate stopping sight distance or warning signs are provided Maximum grade should comply with local standards and criteria; as an example, Delaware uses a maximum grade of 6 percent

Figure 3.6.2. Realigned Intersection in Residential Area
(Source: Google Street View)

EFFECTS AND ISSUES

Effects and Issues – Realigned Intersection
Vehicle Speed	Slows traffic by introducing horizontal deflection to an otherwise straight path of travel (see Figures 3.6.3 and 3.6.4) Can reduce speeds within intersection limits between 5 and 13 mph and between 1 and 6 mph in the vicinity
Vehicle Volume	As a single installation, there is little traffic diversion from the street
Pedestrian Safety and Mobility	Introduction of stop- or signal-control reduces pedestrian/vehicle conflicts and improves pedestrian safety Median island can be placed on the intersection approach and serve as a pedestrian refuge (if its width is at least six feet)
Bicyclist Safety and Mobility	No direct effect on bicyclists
Motorist Safety and Mobility	Likely introduces delay on the major (i.e., realigned) leg of the intersection Minimal impact on motorist comfort
Emergency Vehicle Safety and Mobility	Little effect on emergency vehicle safety or on emergency response time Refer to Module 5 for additional discussion
Large Vehicle Safety and Mobility	Little effect on a large vehicle, provided the realigned curb has an adequate turn radius Refer to Module 5 for additional discussion
Accessibility of Adjacent Property	Should not require the removal of on-street parking
Environment Could be used as a landscaping opportunity
Design Issues	Attention needed to avoid need to relocate drainage features (catch basins, concrete channels, valley gutters, inlets, and trench drains) Should not require relocation of above- and below-ground utilities

Figure 3.6.3. Corner Extension at Realigned Intersection
(Source: Scott Batson)

Figure 3.6.4. Corner Extension at Realigned Intersection (Reverse View)
(Source: Google Street View)

ADDITIONAL DESIGN CONSIDERATIONS

Prior to design of a realigned intersection, a traffic review and capacity analysis should be completed for the intersection. The analyses should determine the appropriate traffic controls for the intersection and determine the extent of queuing and delay (vehicle and pedestrian) changes.

As with any intersection, a typical realigned intersection design needs to consider lighting, drainage, pedestrian safety, pedestrian mobility, and bicyclist access.

The curb extension or narrowing at the intersection should include signs or landscaping that draw attention to it. However, adequate intersection sight distance should be maintained.

Figures 3.6.5 and 3.6.6 illustrate sample designs for realigned intersections with and without approach leg medians, respectively.

Figure 3.6.5. Sample Design for Realigned Intersection with Median Island
(Source: Delaware Department of Transportation)

Figure 3.6.6. Sample Design for Realigned Intersection without Median Island
(Source: Delaware Department of Transportation)

3.7 Traffic Circle (Not Roundabout)

DESCRIPTION AND GENERAL PURPOSE

A traffic circle is a raised island, placed within an unsignalized intersection, around which traffic circulates. A circle forces a motorist to use reduced speed when entering and passing through an intersection, whether the vehicle path is straight through or involves a turn onto an intersecting street.

A traffic circle can have Stop signs or Yield signs on the intersection approaches.

The primary benefit of a traffic circle is an expected reduction in the number of angle and turning collisions. An additional benefit is that it can slow high-speed traffic at the intersection.

[A traffic circle is sometimes called an intersection island.]

A typical traffic circle has a horizontal clearance that is too small for a left-turning truck, emergency vehicle, or bus to circulate counterclockwise even with a partially mountable center island. If the local jurisdiction permits the movement, the large vehicle can make a left turn in front of the island. However, some jurisdictions prohibit turning in front of the island.

A traffic circle is typically designed to fit within the travel lanes of an existing intersection. Because of the infrequent large vehicle turning left on the near side of the circle, the intersection approaches do not have splitter islands (see Figures 3.8.1, 3.8.2, 3.9.1, and 3.9.2 for photographs of splitter islands).

A traffic circle can simply be a painted area, but it is most effective when it is defined by a raised curb and landscaped to further reduce the open feel of a street. A traffic circle can be landscaped with ground cover, flowers, and street trees. Figures 3.7.1 and 3.7.2 illustrate two extremes in the amount of traffic circle landscaping.

A traffic circle is usually circular in shape, but may be oval to fit a particular intersection.

An animated video demonstrating pedestrian, bicyclist, and motorist movements at a traffic circle can be accessed at the following hyperlink: https://www.youtube.com/watch?v=L2WyfWLm53c (Source: City of Winnipeg, Alberta)

[A field study of 45 traffic circles measured an average reduction of 4 mph for 85^th percentile speeds http://www.safety.fhwa.dot.gov/speedmgt/ref mats/eng count/2014/reducing speed.cfm (Source: FHWA, Engineering Speed Management Countermeasures: A Desktop Reference of Potential Effectiveness in Reducing Speed, July 2014)]

Figure 3.7.1. Landscaped Traffic Circle
(Source: Scott Batson)

Figure 3.7.2. Traffic Circle without Landscaping
(Source: Scott Batson)

APPROPRIATE APPLICATION

Appropriate Application – Traffic Circle
Type of Street	Appropriate for the junction of two local roads Can be appropriate in both an urban and suburban setting; see Figures 3.7.3 and 3.7.4 for applications in commercial and residential settings, respectively
Intersection or Roadway Segment	Applicable only at an intersection8 Not typically appropriate for an offset intersection
Roadway Cross-Section	Can be used at intersection of both one-way and two-way streets Preferable for roadway to have urban cross-section (i.e., curb and gutter) Can be applied on a cross-section both with and without a bicycle facility; a bicycle lane is not striped within a traffic circle Can be applied along a roadway with on-street parking
Speed Limit	Requires a slow approach by vehicles; appropriate only for streets with relatively low speed limits; as examples, Delaware and South Carolina use a 30 mph maximum
Vehicle Traffic Volume	Can be an appropriate measure at low traffic volumes; as an example, Pennsylvania uses a daily volume maximum of 3,500 for each intersection leg
Emergency Route	Not appropriate along a primary emergency vehicle route or on a street that provides access to a hospital or emergency medical services
Transit Route	In general, a transit route should not include a left turn at a traffic circle
Access Route	Typically not appropriate along a primary access route to a commercial or industrial site
Grade	Can be installed on a crest vertical curve only if there is adequate stopping sight distance or warning signs are provided Maximum grade should comply with local standards and criteria

⁸ A traffic calming feature placed midblock and in the middle of a road is considered a median island and is treated as a separate measure in this ePrimer

Figure 3.7.3. Traffic Circle in Commercial Setting
(Source: Scott Wainwright)

Figure 3.7.4. Traffic Circle In Residential Setting
(Source: Chris Tzeng)

EFFECTS AND ISSUES

Effects and Issues – Traffic Circle
Vehicle Speed	Traffic speeds within the limits of a traffic circle reduced 5 to 13 mph; in vicinity (i.e., within roughly 200 feet upstream and downstream), a smaller amount (between 1 and 6 mph) Has less of a speed reduction effect than does a small modern roundabout or mini-roundabout because a circle generally does not have splitter islands Most effective in reducing speeds when several are used in a series Refer to Module 4 for additional data
Vehicle Volume	As single traffic calming treatment, has wide-ranging effect on vehicle volume; more effective when placed in series
Pedestrian Safety and Mobility	Depending on geometry of overall intersection, horizontal deflection may force motor vehicle into pedestrian crossing area (see Figure 3.7.5); may be desirable to shift crosswalks slightly in order to prevent vehicles from encroaching on the crosswalk
Bicyclist Safety and Mobility	Bicyclist and motor vehicle share travel lane within the traffic circle (see Figure 3.7.6)
Motorist Safety and Mobility	Minimal impact on motorist comfort Constrained turning radius counterclockwise around circle may necessitate a large vehicle to make a left turn in front of the circle; if turning volume is significant, could create confusion and unexpected vehicle-vehicle conflicts
Emergency Vehicle Safety and Mobility	Fire vehicle can maneuver around traffic circle at slow speed Emergency response times may be affected Constrained turning radius typically necessitates a left turn in front of the circle Refer to Module 5 for additional discussion
Large Vehicle Safety and Mobility	Horizontal deflection could discourage large vehicle operator from using traffic circle if alternative path is available Constrained turning radius counterclockwise around the circle may necessitate a large vehicle to make a left turn in front of the circle Refer to Module 5 for additional discussion
Accessibility of Adjacent Property	Parking should not be permitted close to intersection with traffic circle; distance of 30 feet is commonly used Should not affect accessibility of nearby driveways
Environment	Can be used as a landscaping opportunity
Design Issues	Drainage typically works best if the cross-section slopes away from the traffic circle, creating a reverse superelevation (that in turn can encourage reduced vehicle speeds around the circle) May interfere with and require redesign of access to existing utilities (e.g., a manhole) May require additional street lighting

Figure 3.7.5. Potential Pedestrian-Vehicle Conflict at Traffic Circle
(Source: Scott Batson)

Figure 3.7.6. Bicyclist Passing Through Traffic Circle
(Source: www.pedbikeimages.org / Dan Burden)

ADDITIONAL DESIGN CONSIDERATIONS

The center island of a traffic circle is not traversable.

The center island in the traffic circle should be large enough so that all vehicles are required to follow an indirect path, even to proceed straight through the intersection.

The roadway design vehicle for a traffic circle is usually a passenger car. A larger vehicle may need to cross in front of the traffic circle in order to make a left turn.

A narrow truck apron used at a traffic circle should be designed, at a minimum, to provide through movements for a service vehicle (SU-30).

The center island of a traffic circle should be a different pavement type than the surrounding roadways to increase its visibility. Textured or concrete pavement is commonly used to distinguish the center island from the surrounding pavement.

For a traffic circle, the center island pavement is typically 3 inches above the existing street grade. Traffic circles typically maintain all existing street grades and drainage is not often an issue.

If a traffic circle is designed for a T-intersection, the intersection curb should be either extended at the entrance and exit to the intersection or indented within the intersection to ensure adequate deflection of the vehicle path along the top of the T.

Typical complementary signage is a Yield sign on each approach. It is common for Stop sign control to remain in place at a traffic circle, in particular where Stop signs predate installation of a traffic circle.

3.8 Small Modern Roundabout and Mini-Roundabout (Not Traffic Circle)

DESCRIPTION AND GENERAL PURPOSE

A small modern roundabout and mini-roundabout is a raised island, placed within an unsignalized intersection, around which traffic circulates. The center island forces a motorist to use reduced speed when entering and passing through an intersection, whether the vehicle path is straight through or involves a turn onto an intersecting street. It is also expected to reduce the number of angle and turning collisions.

Both a small modern roundabout and a mini-roundabout are designed in accordance with roundabout design principles. Both are designed so that all traffic can circulate counterclockwise around or partially over the center island.

The principal difference between a small modern roundabout and a mini-roundabout is found at the center island. For a small modern roundabout, the center island is not traversable and can be landscaped with ground cover, flowers, and street trees. In contrast, the center island of a mini-roundabout is fully traversable.

Both a small modern roundabout and mini-roundabout use splitter islands to direct traffic entering the intersection. In order to accommodate trucks, fire trucks, school buses and vehicles towing trailers, the splitter islands can be either mountable or at-grade.

Traffic entering the intersection yields to vehicles within the roundabout.

The reader should reference the Federal Highway Administration (FHWA) report Mini-Roundabouts for a complete description of mini-roundabout characteristics, applicability, and effectiveness. As defined in the FHWA report, "a mini-roundabout is a type of intersection that can be used at physically-constrained locations in place of stop-controlled or signalized intersections to help improve safety problems and reduce excessive delays at minor approaches .... Mini-roundabouts generally have an inscribed circle that is small enough to stay within the existing right-of-way (or within the existing curb lines if adequate space is available). Mini-roundabouts operate in the same manner as larger roundabouts, with yield control on all entries and counterclockwise circulation around a mountable (traversable) [center] island."

A photograph of an example small modern roundabout is shown in Figure 3.8.1.

A video that explains and demonstrates mini-roundabouts can be accessed at the following hyperlink:

https://www.youtube.com/watch?v=94h1TCK-yNs
(Source: U.S. Department of Transportation, Federal Highway Administration)

Figure 3.8.1. Small Modern Roundabout
(Source: Ken Sides)

APPROPRIATE APPLICATION

Appropriate Application – Small Modern Roundabout and Mini-Roundabout
Type of Street	Appropriate for the junction of two local roads or of a local and collector road Can be appropriate in both an urban and suburban setting
Intersection or Roadway Segment	Applicable only at an intersection9 Typically not appropriate for an offset intersection
Roadway Cross-Section	Can be used at the intersection of both one-way and two-way streets Approach legs must be one lane in each direction (two-lane approaches are addressed through the use of a roundabout) Typically preferable to have an urban cross-section (i.e., curb and gutter) but there are many built where shoulder cross-section is transitioned to the roundabout Can be applied on a cross-section both with and without a bicycle facility; a bicycle lane is not striped within a small modern roundabout or mini-roundabout Can be applied along a roadway with on-street parking
Speed Limit	Requires a slow approach by vehicles; either with a relatively low speed limit or other features on the approaches to warn of the roundabout intersection
Vehicle Traffic Volume	Can be an appropriate measure at lower traffic volume levels than a roundabout (see Figure 3.8.2)
Emergency Route	Appropriate along a primary emergency vehicle route or on a street that provides access to a hospital or emergency medical services
Transit Route	Although a transit vehicle can negotiate the turn, in general, transit route should not include a left turn at a small modern roundabout or mini-roundabout
Access Route	Can be applied along a primary access route to a commercial or industrial site
Grade	Can be installed on a crest vertical curve only if there is adequate stopping sight distance or warning signs are provided Maximum grade should comply with local standards and criteria; as examples, Virginia and Portland OR limit longitudinal street grades to 10 percent

⁹ A traffic calming feature placed midblock and in the middle of a road is considered a median island and is considered a separate measure.

Figure 3.8.2. Mini-Roundabout with Splitter Islands
(Source: Omni-Means, Ltd. and Google Street View)

EFFECTS AND ISSUES

Effects and Issues – Small Modern Roundabout and Mini-Roundabout
Vehicle Speed	Speed reduction largely dependent on proper design of approach lanes to deflect each vehicle as it passes through intersection; without adequate deflection, motorists can pass through small modern roundabout and mini-roundabout without lowering vehicle speed FHWA publication Roundabouts: An Informational Guide shows travel speeds approximately 40% lower within mini-roundabout than 350 feet away from intersection
Vehicle Volume	As single traffic calming treatment, there is little traffic diversion from the street
Pedestrian Safety and Mobility	Fewer vehicle/pedestrian conflict points than traditional four-leg intersection Depending on geometry of overall intersection, horizontal deflection may force motor vehicles into pedestrian crossing area on the cross street; may be necessary to move crosswalks further away from mini-roundabout to prevent vehicles from encroaching on the crosswalk
Bicyclist Safety and Mobility	Bicyclist and motor vehicle to share travel lane within mini-roundabout
Motorist Safety and Mobility	Can improve motorist safety at the intersection; has fewer potential vehicle/vehicle conflicts points than traditional four-leg intersection; left-hand turn crashes eliminated Minimal impact on motorist comfort
Emergency Vehicle Safety and Mobility	Turns made smoothly across small modern roundabout apron or mini-roundabout center island Refer to Module 5 for additional discussion
Large Vehicle Safety and Mobility	Lateral deflection for through movements may discourage large vehicle operator from using small modern roundabout or mini-roundabout if alternative path is available Refer to Module 5 for additional discussion
Accessibility of Adjacent Property	Parking should not be permitted close to a small modern roundabout or mini-roundabout; distance of 30 feet is commonly used Should not affect the accessibility of nearby driveways; Virginia recommends a minimum distance of 100 feet to a driveway
Environment	Small modern roundabout can be used as landscaping opportunity; for a traversable mini-roundabout, color or texture treatment can be used or center island can even be branded to relate to a nearby school, neighborhood, business district, etc. (see Figure 3.8.3)
Design Issues	Drainage typically better if cross-section slopes away from center island; reverse superelevation can reduce vehicle speed May interfere with and require redesign of access to an existing utility (e.g., a manhole) May require additional street lighting

Figure 3.8.3. Mini-Roundabout Center Island with Color Pavement
(Source: Dan Burden)

ADDITIONAL DESIGN CONSIDERATIONS

The center island of a small modern roundabout is not traversable. The center island of a mini-roundabout is fully traversable.

The design vehicle for travel around a small modern roundabout or mini-roundabout is the passenger car. A single-unit truck can pass around either but may need to mount the apron of the center island. A larger truck or bus passing through a small modern roundabout may also need to traverse the center island apron. When a larger truck or bus passes straight through a mini-roundabout intersection, it may have to traverse a portion of the center island. When making a left turn at a mini-roundabout, a larger truck or bus is not able to circulate counterclockwise around the center island, but instead travel over the center island at a slow speed (see Figure 3.8.4).

The center island of a mini-roundabout should be a different pavement type than the surrounding roadways to increase its visibility. Textured or concrete pavement is commonly used to distinguish the center island from the surrounding pavement.

For drainage, the circulating lane of a small modern roundabout and mini-roundabout typically slopes away from the center island at a slope between 1 and 2 percent.

If a small modern roundabout or mini-roundabout is designed for a T-intersection, the intersection curb should be either extended at the entrance and exit to the intersection or indented within the intersection to ensure adequate deflection of the vehicle path along the top of the T.

Typical complementary signage is a Yield sign on each approach. The MUTCD has a sample striping layout for a mini-roundabout. A sample design for a mini-roundabout is presented in Figure 3.8.5.

Figure 3.8.4. Mini-Roundabout with Truck
(Source: Ian Lockwood)

Figure 3.8.5. Sample Design for Mini-Roundabout
(Source: Delaware Department of Transportation)

Note: In order for this design to be ADA-compliant, the splitter island on the right-hand leg of this mini-roundabout must be striped and flush with the pavement.