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72 CHAPTER FIVE CONCLUSIONS ⢠More extensive and aggressive measures tended to pro- duce greater reductions in speed and crash occurrence than less extensive and passive measures. ⢠There needs to be a distinct relationship between a settle- ment speed limit and a change in the roadway character. ⢠There is not one particular measure that is appropriate for all situations. Each settlement 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 village. Otherwise, speeds may rebound to previous levels as soon as 820 ft (250 m) from the start of the lower speed zone. With respect to the design of transition zones, there needs to be greater attention to treating the transition zone as a length of highway upstream of the rural to urban threshold, rather than as a specific point (i.e., the threshold itself). A good deal of empha- sis has been placed on creating a gateway to address transition zone issues, and placing the gateway at the location where the character of the road changes from rural to urban in order to build credibility with motorists. However, this places the gate- way at the downstream end of the transition zone, which leaves the transition zoneâthe length of road where the speed change is expected to occurâessentially untreated except for advance speed limit signs or a stepped-down speed limit. In designing and selecting transition zone measures, the ultimate goal is to have motorists traveling at the lower speed limit at the start of the settled area, and to have the speed reduction occur in the transition zone. This places the transi- tion zone in the rural area, and requires the designer to use a variety of techniques to achieve the goal. 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 Figure 39. The basic principle is that motorists are first provided with warning devices and psychological measures in the approach zone, and are faced with physical measures in the transition zone. This approach to transition zone design is intended to bet- ter satisfy driver expectations, and to avoid the abrupt appear- ance of a gateway feature or a physical calming feature. The North American experience with high-to-low speed transition zones is generally disjointed and fairly limited. More than one-half of state and provincial respondents to the state-of-the-practice survey indicated that they do not have a standard method of treating transition zones, and those that do are relying on standard traffic signs as specified in the MUTCD. The experience from overseas is more robust, and the lessons learned from foreign testing might be used as a starting point for developing a North American effort into this important area of research. Measures that have been implemented in transition zones fall into four broad categories: geometric design changes, traffic control devices, road surface treatments, and roadside features. The effectiveness of the various measures is generally determined through the impact these measures have on operating speed or crash risk. Testing methodologies include both driving simulator studies and field experiments. The general trend with these studies is that all but the most aggressive transition zone measures have little or no effect on operating speed, but the effect on crash risk is generally positive. However, these transi- tion zone studies show a significant degree of inter-study variation, which draws into question the transferability of any of results. The variation might be explained by dif- ferences in the applied treatment (e.g., âgatewaysâ hav- ing different designs in different studies), and recording speeds/crashes at different points or over different sec- tions of road. Only a handful of studies examined the long-term or habituation effects of high-to-low speed transition treat- ments. It has long been recognized that change creates uncertainty, and uncertainty may result in lower operating speeds. However, whether the change is sustained over the long term requires further investigation. At the end of the day, the profession requires a more rational approach to experimenting and gathering data on the effectiveness of transition zone measures than the somewhat disjointed trial- and-error method currently being used. The design guidance that is available on transition zones from the literature review for this project (see chap- ter two) is generally consistent in providing the following information:
73 be the most effective measures in managing speeds, may be considered at the downstream end of the transition zone. Finally, at the present time there is an interesting bifurca- tion between the North American and European approaches to speed management and road safety in small towns, settle- ments, and villages. The European community is currently experimenting with minimizing and removing traffic con- trol and design features that physically separate road user types (e.g., removal of directional dividing lines). This is an attempt to create a measure of uncertainty in the driv- ing environment that reduces operating speeds and requires motorists to pay closer attention to the driving task and to communicate (i.e., make eye contact) with other road users. This approach has not been specifically linked to transi- tion zones, but it is in stark contrast to the North American approach to speed management, which has been to add mea- sures. The results of these experiments are very preliminary, but should be monitored. FIGURE 39 Transition zone and approach zone concepts. Having stated that, the speed adaptation phenomenon makes any attempt at transitioning drivers from high to low speeds difficult at best. Because speed adaptation is a car- ryover effect from a previously traversed high-speed area, it is unlikely that infrastructure changes in the transition zone will ameliorate this effect. Moreover, as motorists underes- timate their operating speeds in these situations, it may be beneficial to use speed feedback signs at the upstream end of the transition zone. These signs provide more reliable infor- mation on current operating speeds than the drivers own senses, and can assist motorists in slowing to an appropriate speed. Most of the guidelines and recommendations for tran- sition zones rely on horizontal deflections and narrowings to achieve speed reductions, and specifically recommend against any vertical deflections (i.e., speed humps and raised crosswalks) in the transition area. When using an approach zone and a transition zone, vertical deflections, which tend to
