Human Factors Guidelines for Road Systems: Third Edition (2025)
Chapter: 9 Transition Zones Between Varying Road Designs
PERCEPTUAL AND PHYSICAL ELEMENTS TO SUPPORT RURAL-URBAN TRANSITIONS
Introduction
Rural-urban transition zones occur in areas where high-speed roadways (e.g., in a rural environment) change or transition to low-speed roadways (e.g., in an urban environment with higher development density, higher pedestrian activity, on-street parking). When entering a lower speed zone, particularly after a period of driving at a high speed, drivers generally underestimate their speed and, consequently, do not reduce their speed sufficiently to comply with the lower speed limit. Infrastructure measures can help communicate to the driver the transition from one traffic environment to another and help drivers adjust to the lower speed (1). In designing and selecting transition zone measures, the goal is to have motorists traveling at the lower speed at the start of the settled area, i.e., to have the speed reduction occur in the transition zone (2). Because physical measures are the most effective in reducing speed but are the most perilous if traversed at high speed, it may be helpful to recognize that an approach zone is required upstream of the transition zone to warn motorists. The transition zone and approach zone concept are shown in the figure below.
TRANSITION ZONE AND APPROACH ZONE CONCEPTS
Source: Forbes (2)
Long Description.
The zones, from left to right are Settled area (low speed limit), Transition zone (physical measures), Approach zone (warning and psychological measures), and Rural area (high speed limit).
Discussion
NCHRP Synthesis 412 examined high-to-low speed transitions in North America and overseas (2). While the North American practice relies mainly on traffic signs, the experience from overseas is more varied and includes more extensive and aggressive measures such as geometric design changes, road surface treatments, and roadside features. NCHRP Report 737: Design Guidance for High-Speed to Low-Speed Transition Zones for Rural Highways (3) built upon the literature review conducted in NCHRP Synthesis 412 with a more extensive research project and provided a broader methodology for assessing transition zones and identifying design guidance for potential treatments, including the following (2, 3):
- More extensive and aggressive measures (e.g., mini roundabouts, raised speed tables) tended to produce greater reductions in speed and crash occurrence than less extensive and passive measures (e.g., gateway signing).
- There needs to be a distinct relationship between an urban speed limit and a change in the roadway character.
- There is no single particular measure that is appropriate for all situations. Each facility must be assessed and treated based on its own characteristics and merits.
- To maintain a speed reduction downstream of the transition zone, it is necessary to provide additional measures through the settled area/post-transition zone; otherwise, speeds may rebound to previous levels as soon as 820 ft from the start of the lower speed limit.
- Physical changes to the roadway and roadside are favored treatments because they have permanent and lasting effects. The impacts of enforcement and education programs are more transient and less effective.
- Combinations of treatments are more effective at reducing speeds and improving safety within a transition zone and through a community than a single treatment.
- Consideration should be given to prohibiting passing within the transition zone.
Distortions in speed estimation may occur when a driver must accelerate or decelerate. As a general rule, drivers usually produce a smaller change in speed than that required by the transition. The absolute error is greater after deceleration than after acceleration. This highlights the problems that drivers may face in the high-speed to low-speed transition area. Drivers make larger speed adjustments where the transitional situation is clearly functional (i.e., where the need to adjust the speed of their vehicle is sufficiently “obvious”). Signing alone is not sufficient to induce appropriate speed behavior if it does not correspond to the way in which the driver perceives and categorizes the situation. Discrepancies between the structural elements of the road environment and the posted speed reinforce inappropriate driving behavior (1).
Design Considerations
Current AASHTO policy provides a sufficient level of detail for designing roads in a high-speed environment and a low-speed environment; however, the AASHTO Green Book lacks sufficient guidance on the transition zones between the two facility types (4). Speeds chosen are lower where the height of the vertical elements [e.g., trees, large shrubs, oversized signage (4)] is greater than the width of the road. However, roadside elements need to be chosen carefully so as not to become obstacles that can have a negative effect on safety (1).
Cross References
Speed Perception, Speed Choice, and Speed Control, Chapter 14
Key References
1. Organization for Economic Co-operation and Development (OECD) and European Conference of Ministries of Transport (ECMT). (2006). Speed Management. Paris, France: OECD Publishing.
2. Forbes, G. (2011). NCHRP Synthesis of Highway Practice 412: Speed Reduction Techniques for Rural High-to-Low Speed Transitions. Transportation Research Board of the National Academies, Washington, DC.
3. Torbic, D. J., Gilmore, D. K., Bauer, K. M., Bokenkroger, C. D., Harwood, D. W., Lucas, L. M., Frazier, R. J., Kinzel, C. S., Petree, D. L., and Forsberg, M. D. (2012). NCHRP Report 737: Design Guidance for High-Speed to Low-Speed Transition Zones for Rural Highways. Transportation Research Board of the National Academies, Washington, DC.
4. AECOM Canada Ltd., CIMA+, and Lund University. (2009). International Road Engineering Safety Countermeasures and Their Applications in the Canadian Context. Ottawa, Ontario: Transport Canada.
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