The purpose of transition zones is to reduce speeds incrementally before a vehicle enters a rural community. In order to ensure a high rate of motorist compliance with the intended role of these zones, it is critical to have transition zones that are properly designed with realistic and clearly-posted speed limits. It was noted by Hallmark et al. (2007) that many rural communities set transition zones that either extend much farther past the community limits than are needed, or set transition zone speeds which are otherwise inconsistent with the physical characteristics of the roadway. In other cases, the transition zones are too short/abrupt and do not allow for a comfortable and gradual reduction in speed. For example, Hallmark's study found one rural community with a transition zone that changed immediately from 55mph to 25mph. As discussed in Chapter 2 and Chapter 3, and further detailed in this Chapter, before a speeding countermeasure is implemented within a transition zone, agencies should carefully review the delineation of the transition zones themselves, and ensure they are consistent with good design practice.
4.1 Purpose for Transition Zones
The rural roadway provides high-speed mobility outside the community, yet the same road within town provides local access and accommodates pedestrians, on-street parking, bicyclists, and other features unique to small communities. In cases of poorly delineated transition zones, drivers who have been traveling for some distance on the high-speed road, and are only passing through the community (i.e., not intending to stop for any services or visits), may not receive sufficient visual clues that the character of the roadway is changing, and may not adjust their speeds appropriately. As a result, a proper transition zone provides an area where drivers are alerted that the roadway conditions are changing, in an effort to allow them time to react and slow down to the appropriate speed (Hallmark et al. 2013).
4.2 Transition Zone Layout
The length of a transition zone should be based on a sufficient distance to reduce speeds before drivers enter the rural community. Speed zones around a rural community can be divided into 3 locations (Torbic et al. 2012) as shown in Figure 4-1. The geographic extent of the transition zone is based on distances needed to achieve a speed reduction, depending on posted speed limits and surrounding land use. This information may come from direct observation, discussions with local residents or businesses, or a review of speed and crash data (Torbic et al. 2012).
4.2.1 Rural Zone
Figure 4-1 identifies the rural zone as the open country section of a rural corridor. Design speeds within this zone are usually greater than or equal to 45mph and the roadway is characterized by minimal access and high-speed longer distance travel. Any speeding countermeasures within this area should be consistent with higher speed designs and posted speed limits (Torbic et al. 2012).
Figure 4-1 Speed Zones. (Image Source: FHWA)
4.2.2 Transition Zone
The transition zone is the section of roadway before a motorist enters a community, where drivers need to identify an upcoming change in roadway character, and then complete the necessary speed reduction in preparation for entering the rural community. This area has two distinct components (Torbic et al. 2012).
The first area of the transition zone provides an appropriate distance for the driver to be made aware of the need to adjust speed and driving behavior - otherwise known as the perception/reaction area. The roadway characteristics for the perception/reaction area are similar to those in the rural zone, but consideration should be given to providing clear lines of sight to upcoming speed limit signage or other speed management measures. Speeding countermeasures in this area are provided to alert drivers to changing conditions ahead and provide a warning that conditions are changing so that drivers are able to adjust their speeds. Treatments should be located in the perception/reaction area so drivers are able to adjust.
The deceleration area is where drivers are expected to physically slow to a safe operating speed before entering the rural community. Roadway and roadside characteristics are changing. This area should include treatments to reinforce the need for a speed reduction. The length of the deceleration area is based on factors such as design speed, sight distance, and roadway characteristics.
4.2.3 Community Zone (Town Center)
The community zone marks the developed area of the rural community. By the time drivers reach the community zone, speeds should be reduced and maintained at the in-town speed limit that is set to ensure the safety of all users. The community zone extends from the transition zone on one side of the rural community to the transition zone on the other. Torbic et al. (2012) indicates that signage for the community zone speed limit should be placed near the edge of development, or at a setback of a few hundred feet between the edge of the community and transition zones.
Within the community zone, the posted speed should be appropriate for traffic and roadway conditions. In many cases, development will already extend past these formal community limits. In such cases, the appropriate speed limit should be determined by nature of the surrounding development, density of homes or businesses, or other characteristics that are unique to the rural community. In some instances, it may be difficult to establish what constitutes a rural community. Clusters of homes along a high-speed rural roadway may not necessarily indicate a rural community even when legally defined as a township. While federal-aid language defines a rural community as an area of 5,000 or less population that is "not urban," some additional characteristics for such a community can include several of the following (Torbic et al. 2012):
- Sensitive land use such as schools, recreational areas, and parks.
- Higher land-use intensity.
- Lower design speeds.
- Increased traffic control.
- On-street parking.
- Curbs and gutters.
- Frequent access points.
- Pedestrian and bicycle activity.
- Narrow lanes.
- Turn lanes.
4.3 Selecting and Setting Appropriate Transition Zone Length and Speed
The objective of a transition zone is to gradually reduce speeds from the rural zone to the community zone (or town center). Several agencies have recommended practices for when a transition zone is needed, as shown in Table 4-1. In most cases, the need for a transition zone is determined through an engineering and traffic investigation (Rawson, 2015). Need may also be based on the following factors:
- Roadway operating speeds in advance of speed reduction.
- Safety or operational issues that are due to posted speeds differential.
- History of aggressive braking at entrance to reduced speed limit area.
- Lack of speed compliance in the lower speed limit area.
- Whether motorists are expected to comply with transition speed zone.
4.3.1 Transition Zone Length
The length of the transition zone should be sufficient so drivers are able to comfortably and safely adjust their speed before entering the community. The beginning of the transition zone should not be so far from the town center that drivers do not feel there is a need to comply with the impending speed reduction. Recommendations on transition zone length are provided in Table 4-1. As noted, some agencies have specified a minimum transition zone length (0.2 to 0.3 miles). Wisconsin, by contrast, has a recommend maximum transition zone length.
4.3.2 Transition Zone Speed Limit
Forbes et al (2012) recommends estimating transition zone speed limits by dividing the overall speed reduction by 50 percent (Forbes et al. 2012). For instance, the transition zone along a 60 mph roadway entering a community with a posted speed limit of 30 mph would use a transition zone speed limit of 45 mph. Table 4-1 also provides guidance from other agencies on speed limit setting.
Generally speaking, transition zone speed limits should be consistent with a pre-determined speed metric, such as the 85th percentile speed, or some other limit based on engineering judgment and prevailing roadway characteristics. When actual travel speeds exceed the posted speed limit and have the potential to cause crash or other safety-related issues, speed management and speeding countermeasures should be considered (see Chapter 5).
Table 4-1 Recommended Practices for Rural Community Transition Zone Length and Speed Limit
|Agency||Use||Transition Zone Length||Transition Zone Speed Limit||Source|
|Speed limit difference > 25 mph||Forbes et al. (2012)|
|Texas||Speed limit difference > 15 mph||Minimum: 0.2 miles||TxDOT (2015)|
|North Dakota||Speed limit difference > 20 mph||ND DOT (2015)|
|Wisconsin||Maximum: 0.3 miles||WisDOT (2009)|
|Maine||Minimum: 0.3 miles||Hildebrand et al. (2004)|
|Colorado||Speed limit difference > 15 mph||Hildebrand et al. (2004)|
|Florida and Illinois||Speed limit difference > 10 mph||Hildebrand et al. (2004)|
|Delaware||35 mph for changes 55 to 25 mph||Hildebrand et al. (2004)|
|Virginia||on 65 mph roads, suggests use of 60 mph to 55 mph within or near city limits||Hildebrand et al. (2004)|
4.4 Communicating the Need to Reduce Speeds
The intent of a transition zone is to alert drivers that roadway characteristics are changing and to reduce their speed. To be effective, the delineation of a transition zone needs to be accompanied by a perceived need for the driver to actually reduce their speed.
Torbic et al. (2012) developed design guidance for transition zones and conducted studies on several transition zone treatments. They concluded that transition zone treatments alone are not sufficient and that additional measures need to be provided through the community to maintain a speed reduction. To this need for comprehensive speed management in the transition and community zones, speeding countermeasures that are appropriate for both transition zones and town centers are described in Chapter 5.
4.5 Additional Resources
4.5.1 Methods and Practices for Setting Speed Limits: An Informational Report
Source: http://www.safety.fhwa.dot.gov/speedmgt/ref mats/fhwasa12004/
Year: April 2012
Publisher: Federal Highway Administration (FHWA-SA-12-004) and Institute of Transportation Engineers
Description: The report describes primary practices and methods to set speed limits and includes an engineering approach, expert systems, optimization, and injury minimization. Guidance for setting speed limits is provided and case studies are included. The guidance also discusses speed zones including advisory, school zone, work zones, variable speed limits, and transition zones. This includes guidance for when speed transition are needed as well as setting transition zone speeds.
Year: March 2008
Publisher: US Department of Transportation, Federal Highway Administration
Description: USLIMITS2 is a web-based tool which was designed to assist practitioners in setting consistent and safe speed limits. It is used to set speed limits for specific segments of roads and can be utilized on all types of roads (local roads to freeways).
4.5.3 Speed Enforcement Program Guidelines
Source: https://www.safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa09028/resources/Speed Enforcement Program Guidelines.pdf%23page=1
Year: March 2008
Publisher: US Department of Transportation, National Highway Traffic Safety Administration
Description: The objective of the guidelines is to provide law enforcement personnel and decision makers tools on establishing and maintaining an effective speed management program. The guidelines include:
- Identification of problem.
- Legislative, regulation, and policy.
- Program management including public outreach.
- Enforcement countermeasures.
- Program evaluation.
4.5.4 Speed Concepts: Informational Guide
Year: December 2009
Publisher: Federal Highway Administration (FHWA-SA-10-001)
Description: The guide discusses speed concepts and includes:
- Definitions of speed terms (i.e. 85th percentile speed, design speed).
- Summary of research on the effects of speed.
- Characteristics of speed such as speed distributions and speed profiles.
- Processes to document speeds.
- Agency roles in addressing speed including establishing speed limits and advisory speeds and enforcing speed.
- Speed management technique and countermeasures.
4.5.5 NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 23: A Guide for Reducing Speeding-Related Crashes
Publisher: National Cooperative Highway Research Program, Washington, DC
Description: The guide summarizes collection and evaluation of speed and crash data. It covers strategies to set reasonable and prudent speed limits that account for roadway design, traffic, and environment. The manual also covers increasing driver's awareness of the risks of driving at unsafe speeds.
Forbes, G. T. Gardner, H. McGee, and R. Srinivasan (April 2012). Methods and Practices for Setting Speed Limits: An Informational Report. FHWA-SA-12-004. Federal Highway Administration.
Hallmark, S., E. Peterson, E. Fitzsimmons, N. Hawkins, J. Resler, and T. Welch (October 2007.) Evaluation of Gateway and Low-Cost Traffic-Calming Treatments for Major Routes in Rural communities. Iowa Highway Research Board. http://www.intrans.iastate.edu/research/projects/detail/?projectID=-226410767
Hallmark, S., S. Knickerbocker, and N. Hawkins (January 2013). Evaluation of Low Cost Traffic Calming for Rural Communities - Phase II. Iowa Department of Transportation. Sponsor: Iowa DOT. http://www.intrans.iastate.edu/research/projects/detail/7projectID=43176957
Hildebrand, E., A. Ross, and K. Robinchaud (October 2004). The Effectiveness of Transitional Speed Zones. ITE Journal. pp. 30-38.
MassDOT. MassDOT Policy on Speed Zoning. http://www.massdot.state.ma.us/highway/Departments/TrafficandSafetyEngineering/ PoliciesandDesignGuidelines/ SpeedLimitRegulations.aspx. Massachusetts Department of Transportation, Highway Division. Accessed May 2016.
ND DOT. (September 2015),Speed Limit Guidelines. North Dakota Department of Transportation. http://www.dot.nd.gov/divisions/programming/docs/SpeedLimitGuidelines.pdf
Rawson, C. (August 2015). Procedures for Establishing Speed Zones. Texas Department of Transportation. http://onlinemanuals.txdot.gov/txdotmanuals/szn/manual_notice.htm.
Torbic, D., D. Gilmore, K. Bauer, C. Bokenkroger, D. Harwood, L. Lucas, R. Frazier, C. Kinzel, D. Petree, and M. Forsberg (2012). NCHRP Report 737: Design Guidance for High-Speed to Low-speed Transition Zones for Rural Highways. Transportation Research Board. Washington, DC.
TxDOT. (August 2015). Procedures for Establishing Speed Zones. Texas Department of Transportation.
WisDOT. (June 2009). Wisconsin Statewide Speed Management Guidelines. Wisconsin Department of Transportation. wisconsindot.gov/dtsdManuals/traffic-ops/manuals-and-standards/speed/speed-guide.pdf