Improving Signalized Intersections
FHWA's new guide will help State and local agencies plan, design, and install appropriate facilities to improve safety and traffic operations for all users.
|This intersection features lane-aligned signal heads (one signal head for each lane of traffic), with dual left-turn lanes, two through lanes, and a right-turn lane.|
According to 2002 data compiled by the National Highway Traffic Safety Administration, 21 percent of crashes and 24 percent of all fatalities and injuries related to motor vehicle collisions occurred at signalized intersections. Research conducted by the Federal Highway Administration (FHWA), however, has shown that under the right circumstances installing traffic signals can reduce the number and severity of crashes. But signals that are not designed appropriately can have an adverse effect on safety, so traffic managers need to design, place, and operate them carefully.
Because traffic signals play a key role in enhancing safety, FHWA recently produced a comprehensive handbook that explains methods to evaluate the safety and operation of signalized intersections and that highlights tools to remedy deficiencies. Signalized Intersections: Informational Guide (FHWA-HRT-04-091) provides information and tools that can help traffic engineers, project managers, and other transportation professionals conduct insightful assessments of intersections and understand the tradeoffs from potential improvement measures.
"The state of the art of intersection engineering has been greatly enhanced over the past 10 years, and what the guide does is deploy this new knowledge," says Fred Ranck, a safety design engineer at the FHWA Resource Center, who has taught workshops on intersection design and operation.
The guide includes examples of innovative treatments and best practices used by jurisdictions across the United States. These examples include low-cost measures such as improving signal timing and signs, and more expensive measures such as reconstructing intersections or grade separations. Although some treatments apply only to high-volume intersections, the guide provides solutions relevant to the entire range of traffic volumes.
The guide takes a holistic approach to signalized intersections and considers the safety and operational implications of a particular treatment on all system users, including motorists, pedestrians, bicyclists, and transit users. Also covered are intersection fundamentals, analysis methods, and solutions to intersection deficiencies.
"None of us learns in school how it is in the real world—where the tire hits the road, so to speak—so the guide provides that information," says Thomas Hicks, director of the Maryland State Highway Administration's Office of Traffic and Safety and a member of the committee that reviewed the guide. "You can read in a textbook about reaction time and how wide a lane should be, but the guide puts all the pieces together in terms that reflect what drivers actually see as they drive through an intersection."
|A well-designed signalized intersection can improve traffic safety and mobility. Pavement markings, like those shown in this overhead photo, can be used to delineate travel lanes within wide intersections.|
Designing signalized intersections begins with knowledge of the fundamentals of road user needs, geometric design, and traffic design and illumination, all covered in separate chapters of the guide.
Road users, such as motorists, bicyclists, and pedestrians, are the operative players in the road system, and their perceptions and decisions affect their performance. In the 1980s, FHWA's Human Factors team began applying human factors-based knowledge to the design of roadways and signage. Termed positive guidance, the concept focuses on understanding how road users—primarily motorists—acquire, interpret, and apply information while driving.
The concept of positive guidance is simple: If drivers are provided with the information they need in a format they can read, understand, and react to in a timely fashion, then the chances of driver error will be reduced and safety will be improved.
"The idea is to give motorists the information they need at the time they need it," says FHWA's Ranck. "Intersections are complex meetings of roads, so it is crucial for the driver to get the right information as to what lane to be in and where to go."
Traffic engineers apply knowledge of road user needs by designing and operating signalized intersections that inherently convey to various users what to expect. This information reinforces common expectations or communicates alternative information if uncommon elements are present, such as an emergency vehicle running a red light, allowing sufficient time for drivers to react.
Geometric design of signalized intersections covers channelization principles, number of intersection approaches, intersection angles, horizontal and vertical alignments, corner radius and curb ramp designs, detectable warnings, access control, sight distance, pedestrian facilities, and bicycle facilities. Geometric design includes making evident points of potential conflict in an intersection, particularly those involving vulnerable road users such as pedestrians and bicyclists, and offering the approaching driver, bicyclist, and pedestrian a clear view of one another. For instance, is the intersection free of obstructions such as vegetation, newspaper boxes, and street furniture, such as benches, water fountains, kiosks, clocks, planters, and trash containers?
In addition, the layout of travel lanes, curb ramps, crosswalks, bicycle lanes, and transit stops are all part of the geometric design of a roadway. The design influences roadway safety, shapes the expectations of road users, and defines how to proceed through an intersection. For example, the design can facilitate desired vehicle and pedestrian actions by discouraging undesirable movements, defining appropriate paths for vehicles, encouraging safe speeds, helping separate points of conflict, facilitating the movement of high-priority traffic flows, providing safe refuge, and offering wayfinding clues for bicyclists and pedestrians.
A primary goal of intersection design is to limit the severity of potential conflicts among road users. Intersection channelization, a geometric design concept used to reduce conflicts, employs such techniques as raised medians or traffic islands to discourage wrong-way turns or other undesirable movements. Channelization also uses techniques such as pavement markings to delineate desirable vehicle paths.
Engineers also separate conflict points by adding turn lanes and reducing the number of approaches to the intersection. In addition, intersection approaches that cross as close to 90 degrees as practical can minimize the exposure of road users to potential conflicts.
Pedestrian and bicycle facilities are an important component of geometric design. Pedestrian facilities should be provided at all intersections in urban and suburban areas, and should be designed with the most challenged users—those with mobility or visual impairments—in mind. Among other things, effective curb ramp designs facilitate access to intersections for people with disabilities by enabling access for wheelchairs, scooters, and other wheel-based equipment. Curb ramps also should provide detectable warnings for people with visual impairments, enable boundary identification between the bottom of the curb ramp and the street, and provide a usable grade for equipment accessibility.
Intersections with bicycle lanes or offstreet bicycle paths entering the intersection should be designed to help cyclists navigate the intersection safely. Offroad bicycle facilities (trails) separate bicyclists from other vehicles but are problematic at intersections where trails and roads meet, and drivers may encounter the bicyclists unexpectedly. Onstreet bicycle lanes are better for cyclists at intersections because drivers can expect to encounter them as they navigate through the intersection, but, on the other hand, onstreet lanes can lead to more onroad conflicts.
Proper signalization is a key component in improving the safety and efficiency of intersections. Traffic engineers need to consider a variety of elements when designing a system for signalization at an intersection. One factor is the type of control, either a pretimed signal that operates with a fixed cycle length or an actuated signal that varies the length of the green light based on traffic demand.
Overview of Intersection Traffic Analysis Models
|Project managers use a variety of models, such as those listed in this flow chart, to analyze intersection traffic operations and identify potential countermeasures.
Development of a signal timing plan should address all user needs at a particular location, including pedestrians, bicyclists, transit vehicles, emergency vehicles, automobiles, and trucks. In general, the guidebook recommends that the cycle lengths for conventional, four-legged intersections not exceed 120 seconds.
Another design element is signal phasing. A signal phase is the interval of time allotted for green, yellow, and red in a traffic movement cycle. Signal phasing is the sequence of individual phases in a cycle that defines the order in which pedestrians and vehicles have the right-of-way to move through the intersection.
In split phasing, for example, two opposing approaches move consecutively rather than at the same time (that is, all traffic movements originating from the west followed by all movements from the east). Split phasing can be used in cases where a shared through/left lane is needed, or the geometry of the intersection is such that it is difficult for motorists to make opposing left turns at the same time.
Another important consideration is the layout for the signal poles. Each of the three primary types has advantages and disadvantages:
- Pedestal or post-mounted signals cost less to buy and maintain than other types of signals but do not always meet visibility requirements, particularly at large, high-volume intersections.
- Span wire signals provide flexibility in signal head placement and can accommodate large intersections, but they can suffer wind and ice damage and have higher maintenance costs. In addition, according to FHWA research, some people consider span wire signals aesthetically unpleasing.
- Mast arm signals provide good signal head placement but are more costly than span wire signals, particularly for large intersections.
During the initial stages of signalizing an intersection, project managers determine the scope of analysis needed and collect the appropriate level of data. With this information, they develop a problem statement and identify potential countermeasures or treatments for the intersection. While evaluating the alternatives, they assess possible treatments for feasibility and effectiveness. Once they choose a treatment, they implement and monitor the chosen solution over time.
An important step early in the process is identifying stakeholders and their concerns. Stakeholders include everyone affected by a project, including intersection users, adjacent property owners and residents, and intersection owners and managers.
"Intersection design is complex because it involves much more than just vehicular traffic," says Ranck. "A specific feature such as a large turning radius for heavy trucks, for example, may pose a problem for pedestrians by creating a wide crossing distance."
The following are common concerns raised by stakeholders:
- Motorists—long delays, inefficient signal timing, numerous crashes, poor sight distance, and confusing signs
- Pedestrians—long wait times, wide crossing distance, poorly designed and/or located ramps or push buttons, and confusion about when to begin crossing
- Transit riders—inaccessible or poorly located bus stops, impeded vehicle movement, and difficult traffic stream merges
- Bicyclists—inadequate facilities, ineffective lane striping, and numerous conflicts with vehicles
- Facility managers—meeting of local or State policies, maintaining of traffic flow during construction, and fiscal and right-of-way constraints
The selected performance measures should address the concerns raised by stakeholders as well as the issues identified during the office review and field investigation. Sample performance measures include quantitative measures, such as motorist and pedestrian delays, vehicle queues, approach speeds, and crash severity, and more qualitative measures like multimodal impacts or way-finding success.
Planning for Safety
Safety is a prime consideration for traffic planners when designing, operating, managing, and rehabilitating intersections. Planners have several methods of using collision data to assess intersection safety. Traditionally, engineers have used collision frequency to evaluate the safety of an intersection. Many jurisdictions produce a "Top 10" list of intersections with the highest collision frequency and concentrate their improvement efforts at those sites. Collision frequency, however, does not take traffic volume or collision severity into consideration. By measuring collision rates, which take into account exposure to traffic volumes, engineers can assess the risk that road users face.
By looking at both collision frequency and rates, planners can select those intersections with both high collision frequencies and high collision rates for more detailed safety diagnoses. In addition, they can look at collision severity through use of an index that gives greater weight to collisions resulting in serious injury or fatality than to those resulting in property damage only.
Planners analyzing intersections also use safety performance functions, equations that present the mathematical relationship between collision frequency and volume based on a group of intersections with similar characteristics (that is, signalized, same number of legs, and similar average annual daily traffic). This method can be complex, but it helps planners calculate the potential for safety improvement more accurately than other methods.
The ability to measure, evaluate, and forecast traffic operations is a basic element of effectively diagnosing problems and selecting appropriate treatments for signalized intersections. An analysis of traffic operations describes how well an intersection accommodates demand for all user groups. Planners can use operations analyses at a broader level to determine the size of intersection needed, and at a more refined level to develop signal timing plans.
Planners commonly use three measures of effectiveness to evaluate signalized intersection operations: volume-to-capacity ratio (how well an intersection can accommodate traffic volumes), delay (additional travel time experienced by motorists moving through the intersection), and queue (lines of traffic in areas such as turn lanes).
Remedies for Intersection Problems
Once project planners have analyzed a signalized intersection, they can apply a variety of remedies to minimize safety or operational deficiencies, including systemwide, intersection-wide, alternative, approach, and individual movement treatments.
"The guide describes and illustrates the strengths and weaknesses of various alternatives, and why one would be more appropriate to meet certain needs than another," Ranck says.
|A Florida study found that constructing a median opening that allows drivers to make midblock U-turns reduced crashes more than 26 percent. The diagram shows a right-in/right-out/left-in (RIROLI) intersection and a median U-turn opening located before a signalized intersection. Drivers that desire to turn left out of the RIROLI intersection instead turn right and make a U-turn at the midblock U-turn intersection. Source: FHWA.|
Systemwide treatments apply to roadway segments located within the influence of signalized intersections and to intersections affected by traffic flow along a corridor. These treatments primarily address safety concerns associated with rear-end collisions, turbulence related to vehicles turning in the middle of the block from driveways or nonsignalized intersections, and coordination problems associated with how traffic progresses from one location to another.
An example of a systemwide treatment is construction of a directional median opening to create a midblock opportunity for drivers to make an unsignalized U-turn. A Florida study on this treatment found a reduction in the crash rate of 26.4 percent, compared with direct left turns at the intersection.
Studies have shown that conventional methods of increasing intersection capacity—such as adding left-turn, through, and right-turn lanes—can have diminishing returns. Larger intersections increase traveler delays because of longer clearance times for vehicles and pedestrians, greater imbalances in lane use, and potential queue blockages caused by longer cycle lengths.
Alternative intersection treatments increase capacity and reduce delay by diverting left-turn flows from the main intersection and reducing potential conflicts. An example is the median U-turn crossover, used on some Michigan arterial roads, which eliminates left turns at intersections and moves them to median crossovers beyond the intersection.
For median U-turn crossovers located on a major road, drivers turn left off the major road by passing through the intersection, make a U-turn at the crossover, and turn right at the crossroad. Drivers wishing to turn left onto the major road turn right onto the major road and make a U-turn at the crossover.
Another alternative treatment is the continuous flow intersection, which has been built at a few locations in the United States. Continuous flow intersections eliminate potential conflicts between left-turning vehicles and oncoming traffic by adding a left-turn bay to the left of oncoming traffic. Vehicles access the left-turn bay upstream of the main signalized intersection and cross over the median and the opposing through segment.
Construction of a continuous flow intersection at the junction of Maryland Routes 210 and 228 near Washington, DC, has reduced waiting time at the busy intersection while maximizing its capacity, says Hicks.
"What we didn't want was the southern Maryland traffic not going to Washington stopped at the intersection, so they take a left turn in advance of the intersection," he says. "Because of that, we're able to use two-phase rather than multiphase signals, which reduces the time it takes to get through the intersection."
|Continuous flow intersections, such as this one in Mexico, divert leftturning vehicles to a left-turn bay in the middle of the block.|
An FHWA study of a continuous flow intersection with displaced left turns on all approaches found that average delay was reduced 48 to 85 percent and queue lengths were reduced 62 to 88 percent, compared to a conventional intersection. The effect of this design on intersection operations and safety is still being evaluated, but continuous flow intersections are gaining in popularity.
Approach treatments include sight distance, signing, and pavement markings; grading and intersection angles; and lack of clutter. These treatments ensure that approaching motorists, bicyclists, and pedestrians can see that an intersection is ahead and that a traffic signal is controlling the traffic flow. Adequate signs and pavement markings, for example, help drivers choose an appropriate lane and travel direction. The pavement on the approaches gives drivers a smooth, skid-resistant surface. Sight distance for all approaches should be adequate for drivers proceeding through the intersection, particularly those making left turns.
Individual movement treatments influence how vehicles travel through signalized intersections and how they make left-, right-, and U-turns at those intersections. They help reduce rear-end collisions under congested conditions, collisions involving left-turning vehicles, and bicycle and pedestrian crashes. Examples include adding turn lanes and providing reversible lanes along a roadway section to increase capacity during peak traffic periods without widening the road.
|Using a variable lane sign (as shown here) to add a second right-turn lane during peak traffic periods can increase intersection capacity without widening the road.|
Safer, More Efficient Intersections
Beginning in the spring of 2005, the National Highway Institute will offer a workshop on using Signalized Intersections: Informational Guide to develop intersection projects. The workshop will focus on a case study in which participants assess problems at an intersection and identify alternatives to improve it. To register, visit www.nhi.fhwa.dot.gov.
"By using the guide in the workshop, participants will become familiar with what's in it and how to use it for their own projects," says Ranck.
Assuring the safe and efficient operation of signalized intersections is becoming an increasingly important issue as agencies attempt to maximize vehicle roadway capacity to serve the growing demand for travel. Enhancing safety and reducing crashes are key objectives whenever the design or operational characteristics of a signalized intersection are modified.
"Safety is our number-one goal in Maryland, and mobility is right up there with safety," says Hicks. "We know that good intersection design will improve mobility, and with improved mobility comes enhanced safety."
Joe G. Bared is a highway research engineer in FHWA's Office of Safety Research and Development. He is heading the program area on intersection safety, and manages contracts and conducts staff research in the areas of safety and the operational effects of design. He has a Ph.D. in transportation engineering from the University of Maryland, and he is a registered professional engineer.
Signalized Intersections: Informational Guide will be available from the FHWA Report Center by e-mailing email@example.com, faxing 301–577–1421, or calling 301–577–0818. The guide also is available online at www.fhwa.dot.gov/publications/research/safety/04091/index.cfm.