Design Guidance for High-Speed to Low-Speed Transitions Zones for Rural Highways (2012)

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6S e c t i o n 2 This literature review is divided into three sections. The first section presents key findings from NCHRP Synthesis 412 (Forbes, 2011). The second section provides a summary of exist- ing practice and research, and the third section presents a summary of international guidelines related to transition zones. This section does not provide a detailed account of the transition zone literature. The reader is referred to NCHRP Synthesis 412 for more details. Recent reports by Dixon et al. (2008), Determining Effective Roadway Design Treatments for Transitioning from Rural Areas to Urban Areas on State Highways, and Hallmark et al. (2007), Evaluation of Gate- way and Low Cost Traffic Calming Treatments for Major Routes in Small Rural Communities, also provide a detailed account of the transition zone–related literature. 2.1 Key Findings from NCHRP Synthesis 412 NCHRP Synthesis 412 (Forbes, 2011) was prepared to document what is known about effective and ineffective transition zone design treatments. The focus was on engineering measures used to transition motorists from high- to low-speed areas. The effectiveness of each transition zone treatment was reported in terms of operational (i.e., reductions in speed) and/or safety benefits. In general, the transition zone treatments were grouped into four categories: geometric design, traffic control devices, surface treatments, and roadside features. Forbes reported that most existing transition zone design guidelines are generally consistent in providing the following information: • More extensive and aggressive measures tend to produce greater reductions in speed and crash occurrence than less extensive and passive measures. • There needs to be a distinct relationship between a community speed limit and a change in the roadway character. • No particular measure is appropriate for all situations. Each community must be assessed and treated based on its own characteristics and merits. • It is necessary to provide additional measures through the community to maintain a speed reduction downstream of the transition zone; otherwise, speeds may rebound to previous levels within a distance as short as 820 ft from the start of the lower speed zone. Forbes also reported that there appears to be an emphasis placed on the precise location at which vehicles are expected to be traveling at the lower (i.e., community) speed and the nature of the roadway at this point. However, treatments located along the segment of highway preced- ing this location are critical to alerting the driver of the change in desired speed and the need to provide adequate distance for this transition to take place. Thus, the effects of treatments in com- bination along the entire transition zone length appear to produce the most meaningful results. Literature Review

Literature Review 7 The basic principle is that motorists are first provided with warning devices and psychological measures early in the transition zone, and are then presented with physical measures closer to the community. This approach to transition zone design is intended to better satisfy driver behavior and avoid abrupt appearance of a gateway or physical traffic calming feature. The basic approach to transition zone design is illustrated in Figure 2-1. Finally, Forbes reported on the separate directions that European and North American coun- tries are heading concerning speed management approaches. Several European countries are currently experimenting with minimizing and removing traffic control and design features that physically separate the road users. This is an attempt to create a measure of uncertainty in the driving environment, causing drivers to pay closer attention to the road and reduce their speeds. This is in stark contrast to the North American approach to speed management, which has been to add measures. 2.2 Summary of Existing Practice and Research In addition to reviewing NCHRP Synthesis 412, other resources were reviewed to identify geometric, streetscape, and traffic engineering treatments that demonstrate an effective and safe reduction in speed and are potentially applicable for use in the design of transition zones for rural highways. In addition, a survey was conducted to create an inventory of practical speed reduction treatments for implementation in transition zones. This section combines the results of the literature review and survey and presents a catalog of potential treatments for inclusion in design guidelines for high- to low-speed transition zones. The catalog of potential treatments is divided into groups consisting of geometric design, traffic control devices, pavement surface, and roadside treatments and is presented in Figures 2-2 through 2-5. Figure 2-6 presents information on gateway treatments (i.e., combinations of transition zone treatments). For each transition zone treatment in the catalog, information regarding effectiveness in reduc- ing speeds and/or improving safety is provided (when available). The bold values presented in Figure 2-1. Transition zone and approach zone concepts (Forbes, 2011).

8 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Figures 2-2 through 2-6 are effectiveness estimates as summarized in Chapter 4 of NCHRP Syn- thesis 412 (Forbes, 2011). Although not specifically stated in the synthesis document and chapter, it is implied that the effectiveness estimates presented in Chapter 4 are the most reliable effective- ness estimates for the respective speed reduction treatments. The other effectiveness estimates in Figures 2-2 through 2-6 (i.e., non-bold text) are based on findings from the literature review conducted as part of this research. The reliability and/or accuracy of the documented effectiveness estimates in Figures 2-2 through 2-6 are rated using a star rating system developed as part of this research. The star rating system takes into consideration the robustness of the data supporting the estimates of effective- ness, the appropriateness of the analysis methods, applicability to U.S. conditions, whether the results are based upon field data or simulation, and applicability to transition zones in rural areas. A star rating system from one to four is used, with one star (,) representing “least reliable” and four stars (,,,,) representing “most reliable.” No formal procedure was developed to calculate the star rating for a given effectiveness estimate; rather, the rating was based on a qualitative assess- ment of the factors listed above. Figures 2-2 through 2-6 provide a definition or description of a specific treatment and infor- mation on the expected effectiveness of the given treatment in reducing speed or improving safety. The figures do not include speed reduction treatments considered inappropriate for use in high- to low-speed transition zones. For example, treatments involving vertical deflections of the pavement surface such as speed humps or raised intersections are not included. Introduc- ing such measures in a transition zone has too much potential to lead to loss of vehicle control; therefore, they are considered inappropriate for this type of application.

Treatment: Roundabout Description: A roundabout is a form of circular intersection in which traffic travels counterclockwise (in the United States and other right-hand traffic countries) around a central island. Entering traffic must yield to circulating traffic. The channelized approaches and geometry induce reduced travel speeds through the circular roadway. Speed Estimate: Rodegerdts et al. (2007, 2010) developed speed prediction models for roundabouts: . 3 2b 2 b 3exit d8.13)aR47.1(47.1 1 ;aRMINV 1 2b 2 b 1enter d4.8)aR47.1(47.1 1 ;aRMINV where: Vexit = predicted exit speed (mph) Venter = predicted entry speed (mph) d1 = distance between point of interest on the entry and midpoint of path on circulating roadway (ft) d2 = distance between point of interest on the entry and the midpoint of path on the circulating roadway (ft) d3 = distance between the midpoint of path on the circulating roadway and point of interest on the exit (ft) R1 = path radius on entry to roundabout (ft) R2 = path radius on circulating roadway (ft) R3 = path radius on exit from roundabout (ft) a,b = regression parameters Speed Prediction Parameters: Superelevation (e) = +0.02 Superelevation (e) = –0.02 a 3.4415 3.4614 b 0.3861 0.3673 Source: Rodegerdts et al., 2010 Safety Estimate: Converting a two-way stop-controlled intersection to a roundabout reduces total crashes by 71% and fatal and all injury crashes by 87% (AASHTO, 2010) . Converting a signalized intersection to a roundabout reduces total crashes by 48% and fatal and all injury crashes by 78% (AASHTO, 2010) . 10% to 40% reduction in injury crashes (Elvik and Vaa, 2004) . Figure 2-2. Catalog of potential transition zone treatments and their expected speed reduction and safety effects (geometric design treatments). Treatment: Chicane or Horizontal Deflection Description: Chicanes incorporate the use of pavement markings, planting strips, on-street parking, etc., to create a sequence of horizontal curves (i.e., horizontal deflections) intended to slow vehicles. Speed Estimate: A gateway treatment with and without horizontal deflections reduced speeds of the same magnitude (Lamberti et al., 2009) . Reduced 85th percentile speed from 31 to 28 mph (Macbeth, 1998) . 6% reduction in 85th percentile speeds (Corkle et al., 2001) . 85th percentile speed reduced from 45 to 35 mph. Reduction in mean speed from 36 to 29 mph. Number of vehicles traveling 10 mph above speed limit reduced from 35% to 3% (Hallmark et al., 2007) . Reduced speeds by 26% (Charlton and Baas, 2006) . Safety Estimate: No data found. Source: San Francisco Municipal Transportation Agency (Forbes, 2011) Treatment: Bulbout/Curb Extension Description: At an intersection, the curb line is extended into the street to effectively reduce the street width. By reducing the street width, the pedestrian crossing distance is reduced. Speed Estimate: Narrowing street width at the intersection reduced speeds by 2.6 mph (Ewing, 1999) . Safety Estimate: No data found. Source: Peter Lagerway (Harkey and Zegeer, 2004)

10 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Treatment: Center Island/Raised Median Description: A channelizing island that creates separation between the two opposing directions of travel. Center islands/raised medians can create shifts or deflections in the travel paths of vehicles and often reduce the effective widths of the roadways. Center islands/raised medians can be created through a combination of pavement markings, raised curbs, planting strips, etc. Speed Estimate: Berger and Linauer (1998) developed speed prediction models for center islands: . V85 = 9.194Ln(L/2d) + 12.290 Vm = 8.020Ln(L/2d) + 11.031 where: V85 = 85th percentile speed (mph) Vm = mean speed (mph) L = length of island + length of both tapers (ft) d = lateral deflection of lane (ft) Reduced 85th percentile speeds up to 9.6 mph (Dixon et al., 2008) . 17% reduction in vehicles exceeding the speed limit (Forbes and Gill, 1999) . Median resulted in 3.4-mph reduction in mean speed (Dixon et al., 2008) . Source: Adapted from Berger and Linauer (1998) Median with gateway reduced mean speeds by 10.2 mph and 85th percentile speeds by 5.6 mph (Dixon et al., 2008) . Median with pedestrian crosswalks reduced mean speed by 10 mph and 85th percentile speeds by 5.6 mph (Dixon et al., 2008) . Median with tubular markers reduced speeds by up to 3 mph (Hallmark et al., 2007) . Painted median had no significant effect on operating speeds (Hallmark et al., 2007) . Median reduced speeds by 9% (Charlton and Baas, 2006) . Reduced speeds by 3 mph (Harvey, 1992) . Center islands and curb extensions reduced speeds by 5 mph; center islands alone reduced speeds by 2 mph (TAC, 1998) . Raised medians with signage reduced 85th percentile speeds by 9 mph; signage alone reduced 85th percentile speeds by 6 mph (Crowley and MacDermott, 2001) . Safety Estimate: No data found Treatment: Roadway Narrowing Description: Can be achieved either by physically reducing the roadway width or by narrowing the widths of the travel lanes. This technique is often installed in conjunction with adding bicycle lanes or adding a raised median. Speed Estimate: 2.6-mph reduction in speeds was achieved using a variety of roadway narrowing strategies (Ewing, 2001) . 11% to 20% reduction in operating speeds (Charlton and Baas, 2006) . Safety Estimate: No data found. Treatment: Bicycle Lanes Description: A portion of a roadway designated by striping, signing, and pavement markings for the preferential or exclusive use of bicyclists. Bicycle lanes are often installed in conjunction with roadway narrowing and indicate a change in environment (i.e., increased bicycle activity). Speed Estimate: No data found. Safety Estimate: No data found. Figure 2-2. (Continued).

Literature Review 11 Treatment: Road Diet Description: A reduction in the number of through lanes (e.g., converting a four-lane road to a three-lane roadway with a two-way left-turn lane or converting a four-lane roadway to a two-lane roadway with a raised median or on-street parking). Bicycle lanes are often installed in conjunction with road diets. Speed Estimate: 5-mph or less reduction in operating speeds, but up to 70% reduction in excessive speeding (Knapp and Rosales, 2007) . 5-mph reduction in 85th percentile speeds and 60% to 70% reduction in vehicles traveling more than 5 mph over the posted speed limit (Knapp and Geise, 2003) . 4-mph reduction in mean and 85th percentile speeds (Corkle et al., 2001) . Safety Estimate: 20% to 40% reduction in crashes (Knapp and Rosales, 2007) . 17% to 62% reduction in crashes (Knapp and Geise, 2003) . 29% reduction in crashes (FHWA, 2012) . Sample Road Diet Key to Star Ratings for Reliability of Speed and Safety Estimates: —least reliable —most reliable Figure 2-2. (Continued). Treatment: Transverse Pavement Markings Description: Pavement markings placed perpendicular to the direction of travel to draw attention to a change in the roadway environment. Often the markings are placed in decreasing intervals to give the illusion of increasing speed. Speed Estimate: 3- to 9.5-mph reduction in speed (Arnold and Lantz, 2007) . 1.0-mph reduction in 85th percentile speed (Fitch and Crum, 2007) . Up to 2-mph reduction in 85th percentile speeds (Hallmark et al., 2007) . 3- to 5-mph reduction in mean speeds and 5- to 7-mph reduction in 85th percentile speeds for painted chevrons (Hallmark et al. , 2007) . 3- to 7-mph reduction in 85th percentile speeds from transverse pavement markings with speed feedback signs (Hallmark et al., 2007) . Up to 4-mph reduction in 85th percentile speed with painted chevron and speed marking (Hallmark et al., 2007) . Transverse lines reduced speeds by 8% to 14% (Charlton and Baas, 2006) . Safety Estimate: No data found. Treatment: Lettered Pavement Markings Description: Pavement markings with lettered descriptions such as “Slow.” Speed Estimate: Speed pavement marking with red background reduced 85th percentile speeds by up to 9 mph (Hallmark et al., 2007) . Speed markings on pavement inside of elongated circles (speed roundels) reduced mean speeds on 40-mph approaches by 3 mph. Roundels on 30-mph approaches had no significant effect (Barker and Helliar-Symons, 1997) . Safety Estimate: No data found. Source: Hallmark et al., 2007 Figure 2-3. Catalog of potential transition zone treatments and their expected speed reduction and safety effects (traffic control device treatments).

12 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Treatment: Speed-Activated Feedback Sign Description: A variety of warning and/or dynamic speed signs that indicate a reduction in the speed limit and alert the driver that he/she is traveling above the posted speed limit for that portion of roadway. Speed Estimate: 6-mph reduction in mean speeds (Donnell and Cruzado, 2008) . Mean speed reduction of 4.3 mph (Farmer et al., 1998) . Reduced average mean speeds by 5.4 mph, also found to be capable of reducing the number of drivers who exceed the speed limit by up to 80% (Winnett and Wheeler, 2002) . Reduced speeds by 11% (Charlton and Baas, 2006) . 6.9-mph reduction in the 85th percentile speeds (Sandberg et al., 2006) . 7-mph reduction in 85th percentile speed (Hallmark et al., 2007) . 3% to 12% reduction in mean speeds and 1% to 14% reduction in 85th percentile speeds (Russell and Godavarthy, 2010) . 4% to 11% reduction in mean speeds and 3% to 15% reduction in 85th percentile speeds for mobile speed trailers (Russell and Godavarthy, 2010) . Safety Estimate: 34% reduction in casualty crashes (Winnett and Wheeler, 2002) . Key to Star Ratings for Reliability of Speed and Safety Estimates: —least reliable —most reliable Figure 2-3. (Continued). Treatment: Transverse Rumble Strips Description: Rumble strips placed in the travel lanes perpendicular to the direction of travel to alert drivers of a change in the environment. As motor vehicle tires pass over the rumble strips, drivers receive auditory and tactile warnings to reduce their speeds. Speed Estimate: Rumble strips reduced the mean speed by 1.3 mph but the result was not statistically significant (Ray et al., 2008) . Rumblewave surfaces were responsible for speed reductions of 1% to 6% (Department for Transport, 2005) . Up to 6% reduction in operating speeds (Charlton and Baas, 2006) . Safety Estimate: Rumblewave surfaces experienced a 55% reduction in fatal and injury crashes (Department for Transport, 2005) . Source: Corkle et al., 2001A Treatment: Colored Pavement Description: The use of colored pavement to delineate the functional space of the roadway and to alert drivers of a change in the environment. Speed Estimate: Up to 17% reduction in mean and 85th percentile speeds (Russell and Godavarthy, 2010) . Safety Estimate: No data found. Source: Russell and Godavarthy (2010) Key to Star Ratings for Reliability of Speed and Safety Estimates: —least reliable —most reliable Figure 2-4. Catalog of potential transition zone treatments and their expected speed reduction and safety effects (pavement surface treatments).

Treatment: Welcome Sign Description: A physical landmark or sign that indicates to drivers that they are entering a community. Welcome signs are generally placed to the right of the roadway, but some may be large enough to span the roadway. Speed Estimate: Reduced 85th percentile speeds by up to 3 mph (County Surveyor’s Society, 1994) . Safety Estimate: Are not detrimental to safety (Veneziano et al., 2009) . Treatment: Layered Landscaping Description: Roadside landscaping is provided to enhance the aesthetics of the roadside environment and to increase driver awareness of the environment. Plants are grouped according to height, with smaller plants (i.e., ground cover) placed closer to the roadway and taller plants (i.e., trees) placed further from the roadway. Speed Estimate: Layered landscaping did not result in statistically significant speed reductions (Dixon et al., 2008) . Safety Estimate: No data found. Source: Transit New Zealand (2006) Key to Star Ratings for Reliability of Speed and Safety Estimates: —least reliable —most reliable Figure 2-5. Catalog of potential transition zone treatments and their expected speed reduction and safety effects (roadside treatments). Treatment: Gateway Description: A combination of treatments installed to indicate a change in environment (e.g., welcome sign, roadway narrowing, raised median, bicycle lane, roundabout, etc.). Speed Estimate: Gateways on the approach to a community reduced 85th percentile speeds by 3 to 10 mph (County Surveyor’s Society, 1994) . 4.2-mph reduction in mean speed and 7.2-mph reduction in 85th percentile speed (Pyne et al., 1995) . 5.5-mph reduction in mean speed and 3.0-mph reduction in 85th percentile speed for gateway used with lane narrowing (Dixon et al., 2008) . Gateways including a variety of measures resulted in a 6.9- to 10.6-mph reduction in mean speeds (Lamberti et al., 2009) . A variety of gateway treatments resulted in an 11% reduction in mean speeds and 15% reduction in motorists traveling over the posted speed limit (Herrstedt et al., 1993) . 6.2-mph reduction in average speed 6 months after installation; 3.3-mph reduction in average speed 12 months after installation (Alley, 2000) . 0% to 3% reduction in speeds for gateways with signing and pavement markings. 7.5% reduction in speeds for gateways of high visibility. 15% to 27% reduction in speeds for gateways with high visibility and physical features (Charlton and Baas, 2006) . Safety Estimate: Gateways with combined physical and visual measures reduced injury crashes by 28%, but increased property-damage only crashes by 36% (Andersson et al., 2008) . 55% reduction in fatal and serious injury crashes and 19% reduction in all injury crashes (Wheeler and Taylor, 2000) . Key to Star Ratings for Reliability of Speed and Safety Estimates: —least reliable —most reliable Figure 2-6. Catalog of potential transition zone treatments and their expected speed reduction and safety effects (gateway treatment).

14 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways 2.3 Summary of International Guidelines Internationally, the use of transition zone treatments is more advanced than in the United States. Several countries have developed guidelines for the use of transition zone treatments along rural routes approaching communities. 2.3.1 Australia/New Zealand In Australia, speed has been identified as a major factor in the occurrence and severity of roadway crashes. Austroads, an association of Australian and New Zealand road transport and traffic authorities, funded research to identify and review different approaches to speed manage- ment on rural roads, with a particular focus on engineering-based treatments (Turner, 2008). A number of treatments were assessed at locations with transitions from high to low speeds, including the following: • Advance warning, • Buffer zones, • Countdown signs, • Transverse rumble strips, • Pavement numerals/speed limit markings, and • Rural thresholds/gateway treatments. When used in isolation, these treatments appeared to have limited benefits in reducing speeds and improving safety; however, when used in combination, treatments were found to have sig- nificant effects (e.g., combinations of signs and pavement markings). Guidelines published by the Land Transport Safety Authority (LTSA) (LTSA, 2002) outline the principles of rural-urban speed reduction treatments and promote good design practice and consistency. Given the clearly established correlation between vehicle speeds and crash sever- ity, and the increased exposure to risk, the guidelines seek to provide a framework to reduce traffic speeds to appropriate levels at the outer fringes of urban areas. The focal treatment in these guidelines is referred to as a threshold. Thresholds are defined as locations at boundaries between rural and urban areas and consist of physical and optical narrowing of the roadside to form “pinch points.” When designed correctly, thresholds lead to a reduction in vehicle speeds, as drivers perceive a change in the road environment ahead. Figure 2-7 provides an example of threshold pavement markings. Vertical design features are recommended at transition zones because they improve the vis- ibility of the threshold for approaching drivers. Examples of vertical elements include trees and shrubs, combined speed restriction and place name signs, and the structures or poles that support these signs. Street lighting can also be incorporated as a vertical element. Trees, lighting poles, and poles used to support signs in the threshold area must be frangible (i.e., breakaway) to reduce injury risk to occupants of errant vehicles. Research indicates that drivers travel at a reduced speed where the height of vertical features is greater than the width of the street (LTSA, 2002). Figure 2-8 illustrates the relationship between optical width, optical height, and vehicle speed. Unless thresholds stand out from the surroundings, road users may not notice the approaching change of environment and fail to reduce speed as required. In addition to the vertical and hori- zontal elements described above, there are a number of measures that can enhance conspicuity such as the following: • Brightly colored flowers or shrubs as part of the landscaping, • Trees or shrubs that contrast in color with the surrounding landscape,

Literature Review 15 • Colored paving materials through the threshold pinch point, and • Size and color of the combined speed limit and place name sign. Nighttime conspicuity of the threshold can be enhanced by using reflectorized paint or reflec- tive strips on all curbs used in build-outs or median islands. Australian research also indicates that the use of speed humps or vertical roadway shifts within rural thresholds is not recommended. Research indicates these vertical deflections of the road- way “could create a safety hazard that would cause many more problems than existed previously” (Schnull and Lange, 1994). Figure 2-9 and Figure 2-10 illustrate an untreated and a treated approach to a community. Figure 2-7. Pavement markings and RRPMs without solid islands (RRPM = retroreflective pavement markings) (LTSA, 2002).

16 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways Figure 2-9. Typical untreated approach to a community (LTSA, 2002). Figure 2-8. Optical width (Devon County Council, 1992).

Literature Review 17 Figure 2-10. Completed threshold (LTSA, 2002). 2.3.2 United Kingdom A leading document in the United Kingdom (UK), Traffic Calming Techniques (IH and TCS, 2005), provides updates to designers on the current state-of-the-practice for traffic calming, including guidance on when treatments are appropriate, design and implementation practice, currently used and successful treatments, and the future of traffic calming designs. According to the guidance, successful design is more likely to be achieved by adopting an overall “package” approach, rather than by using a scattering of different measures. This will help avoid one of the most common sources of objection to traffic calming, which is that treatments appear to be ill thought out and the result of a formulaic approach. Guidelines in the UK focus on traffic calming treatments in general, rather than their specific use in transition zones. To determine which designs are most applicable to manage speeds, the UK guidelines recom- mend the analysis of the following roadway data: • Crash Information: – Rates – Types – Causes • Vehicle Flow: – Speed – Composition • Physical Roadway Characteristics: – Widths – Alignment – Pavement markings – Signing – Visual condition – Street lighting – Locations of bus stops – On-road parking – Special features for vulnerable road users – Storm sewers/ditches • Perceived Risk • Road Hierarchy: – Existing road function – Intended road function – Local distribution routes – Public transportation corridors – Pedestrian networks – Cycling networks – Emergency vehicle routes – Abnormal or hazardous materials routes • Environment: – Traffic noise – Ambient noise – Vegetation – Buildings • Uses for the street (consideration of the different kinds of activities that may take place).

18 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways The UK guidelines indicate traffic calming treatments need to be visible both day and night, as well as in adverse weather conditions. The effects of traffic calming treatments on drainage and along steep grades is also an important consideration, as some treatments may adversely affect the driving environment. Traffic calming treatments used in the UK are grouped into the following categories: Road Narrowings, Footway Build-Outs, and Chicanes Roadway narrowing can take the form of the following: • A gateway feature that retains two-way flow of traffic. • A pinch point where traffic can only pass through the feature in one direction at a time. • Central islands to prevent passing. • Chicanes to force traffic to deviate from a straight throughpath. • Reduced width over a length of road. • Reduced width over a length of road with the use of bicycle lanes to visually narrow the street. • Reduced width over a length of road such that smaller vehicles can pass each other but larger vehicles have to stop and let other vehicles proceed. Speeds can be reduced by up to 5 mph using these various road-narrowing techniques. Chicanes have been found to lower speeds to around 20 mph, but this depends greatly on the path angle created. Some disadvantages of these road-narrowing techniques found in UK research include the following: • When features built into the roadway are not conspicuous, drivers may crash into the features or overrun curbs. • If traffic speeds remain high, pedestrians and bicyclists may still feel vulnerable using the features created for them. • Narrow roadways can be difficult for large vehicles to negotiate. • Some drivers may encroach on the opposing lanes to avoid slowing down. Alerting Measures (Rumble Devices and Surface Features) These treatments cause an audible or visual effect to alert drivers to slow down. Rumble strips can be installed in new pavement, ground in, or built up. “Rumblewave Surfacing” is a new product developed with the intention of providing an optimized, sinusoidal profiled surface that generates significant horizontal vibrations in a passing vehicle but minimal external noise. Color patches can be useful for visually alerting drivers to slow down. These can be solid painted areas or transverse pavement markings. Disadvantages of this treatment include the following: • Some people view them as intrusive. • They can wear quickly. • There is little hard evidence to demonstrate their effectiveness. Parking Management and Control By permitting parking along a roadway, the effective driving space can be reduced, causing vehi- cles to slow down. This treatment can be especially effective when combined with the narrowing of traffic lanes or the introduction of chicanes. Gateway Features Gateways are structures built on the sides of a roadway to alert drivers that they are entering a specially designed area where a new speed limit is in place. The gateway feature might be a gate, fence, wall, or even a work of art. It is important that consideration be given to safety, mainte- nance, and suitability of the feature to the local environment. Experience suggests that the speed reduction achieved by a gateway alone is not likely to be significant unless used in combination with other measures, or it is particularly conspicuous.

Literature Review 19 Figure 2-11. Highway sign in the UK warning drivers of the risks of speeding (IH and TCS, 2005). Lighting for Traffic Calming Schemes The necessity for, and design of, appropriate street lighting needs to be carefully considered wherever traffic calming measures are being installed. A lamp source giving good color rendering (i.e., distinction between colors) is necessary to assist the motorist in safely navigating the traffic calming feature, especially when the calming measure involves the use of color. Speed-Activated Feedback Signs The purpose of installing a speed-activated feedback sign is to warn drivers of a change in the environment ahead, or to remind them of a speed limit in force. UK guidelines indicate that such signs should be used in addition to, not instead of, conventional signing. The guidelines indicate that sites should be considered for the use of interactive signs if the following: • There is a recent history of crashes in which inappropriate speed was a factor. • Excessive speed for the conditions (i.e., approaching intersections or curves) is a concern. • Other crash remedial measures have been considered and found unsuitable. Advertising and Publicity To raise awareness of road safety and inform the public of safety concerns at sites, it may be effective to carry out an advertising campaign. Figure 2-11 shows a temporary sign erected to advertise the concerns of excessive speed. 2.3.3 Germany A European Conference of Ministers of Transport (ECMT) document on speed management (ECMT, 2006) addresses guiding principles on transition zone design in Germany. When enter- ing a lower speed zone, in particular after a period of driving at a high speed, the document states that drivers will generally underestimate their speed and consequently not reduce their speed enough to comply with the lower speed limit. Two principles to consider in the design of transi- tion zones are as follows (European Transport Safety Council [ETSC], 1995): • Measures at the transition zone should be such that they achieve a cumulative effect, finishing at the actual gateway to the community. • Complementary measures along the through-route within the urban area are required.

20 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways A cumulative effect can be achieved by a combination of road narrowing and the introduction of trees and other vertical elements, culminating in the gateway. Chosen speeds are found to be lower where the height of the vertical elements is greater than the width of the road. However, the vertical elements need to be chosen very carefully so as not to become roadside obstacles which can have a negative effect on safety. German research has found that, if applied in a consistent way, infrastructure-based measures can help to reduce speeds and help drivers to recognize the traffic situation and comply with the corresponding speed limit. Nevertheless, guidelines recommend that speed management involv- ing road engineering changes should be accompanied by education, information, and enforce- ment to make road users aware of the posted speed, the speeding problem, and the “why” and “what” of countermeasures to increase incentives for compliance. Constructions similar to medieval gates have also been used to indicate the change from one traffic environment to another. In today’s driving environment, the border between the com- munity and the country is less distinct, and this is one of the reasons why many drivers ignore the local speed limits. Gates are used to indicate the beginning of an area where new traffic behavior, and especially a new speed, is required. Figure 2-12 shows such a gate in Germany. Gates may be in the form of an actual building structure, as in ancient times, but they may also be constructed using different forms of plantings, lighting, and the like. Gates improve drivers’ understanding of the different traffic behavior required by delineating the start of a different road design on the same stretch of road. If there is a bicycle lane through the city, it should desirably start at the gate. The change from two lanes into a single roadway, or the narrowing of a lane should also preferably start at the gate location. Speed reductions at these locations depend on the treatment design and changes in road design between previous and following road sections, as well as on the neighboring environment. The speed reduction effect is greatest if the alignment requires a distinct steering maneuver where both visual elements and other traffic calming measures, such as a change in the road profile, road surface, or others are utilized. Other useful treatments for slowing vehicles at transition zones are central islands and round- abouts. Central islands can be one-sided (i.e., divert only the path of incoming vehicles) or two-sided (i.e., divert both incoming and outgoing vehicles). Two-sided islands are preferred, as these prevent vehicles exiting the community from speeding up prematurely. The influence Figure 2-12. Gate into a German community (ECMT, 2006).

Literature Review 21 of central islands on speed has been found to be only moderate, but in addition to speed reduc- tion they also provide the opportunity to build a new, more attractive, urban space by dividing the old street profile. Roundabouts in Germany have been found to provide very effective speed reduction at at-grade intersections. In general, German research indicates that the aim of these measures should be to produce safe, “self-explaining” roads, where drivers recognize the type of road and are guided to adapt their speed to the local conditions. The purpose of German design guidelines for urban roads is to enforce speed reduction at the entrances into urban areas by using several treatments to slow down motorists. 2.3.4 Netherlands Safe Road Design—A Practical Manual (World Bank and Dutch Ministry of Transport [WBDMT], 2005) is a document summarizing speed reduction treatments in the Netherlands. One common treatment to decrease speeds through built-up areas is to create a clear boundary at the community. The guidelines indicate that the main requirements for the boundary of the community center are as follows: • The border of the built-up area is characterized by consecutive buildings alongside the road, with such a size and density that the road user notices a considerable difference between the road environment inside and outside the built-up area. • At the location of the border, there must be a significant change in road characteristics such that the difference in character of the road before and after the border is emphasized as much as possible. Often, the border is in an uninhabited area, so it is not surprising that drivers ignore the speed limit. The clearer it is that the road and the environment have the character of a built- up area near the border, the less the need for specific traffic calming measures and the greater is the driver’s understanding. An example of a community border treatment is presented in Figure 2-13. The Dutch recommend the following guidelines for determining if a particular area should be considered a built-up area: • The distance from the buildings to the centerline of the road is, at maximum, three times the height of the adjoining buildings with a maximum of 82 ft. Border of village before reconstruction Border of village after reconstruction Figure 2-13. Sample community border treatment (WBDMT, 2005).

22 Design Guidance for High-Speed to Low-Speed transition Zones for Rural Highways • The length of the built-up area is at least 1,300 ft. • Building density (i.e., building frontage related to road length) should be ≥50 percent on one side of the road and ≥30 percent for buildings on both sides. In determining the precise location of a border (transition zone), the following aspects need to be considered: • Select a location (preferably) where the different characteristics of the landscape join. • Take account of short-term spatial developments. • Try to support the border with new environmental characteristics. • Ensure the border, at the planned location, is visible at the actual approach speeds. Dutch guidelines recommend the following treatments to make drivers aware of community boundaries: • Replace an intersection before the border with a roundabout. • Realign lanes outward on both sides to create a central island. • Introduce curves suitable for a speed of approximately 30 mph for passenger cars. • Install a 30-mph plateau, a raised crossing area with slopes designed to be crossed at a maxi- mum of 30 mph. An additional Dutch guide, Sustainable Safety—A Preventative Road Safety Strategy for the Future (AVV, 2001), provides recommendations for transitioning from one roadway class to another. These transition zones are termed “gateways” and specific guidance is given depending on the types of roadways between which the transitions are to occur. According to the guide, “If the transition is between a distributor and an access road, then a minimum requirement is a double transverse line over the width of the road and supported by 20-mph (30-km/h) zone road signs on either side. Where the nature of the area is not self-evident, additional measures must be implemented.” Figure 2-14 illustrates a transition between a rural road and an urban area with a 20-mph (30-km/h) speed limit. Figure 2-14. Gateway treatment—rural distributor to 30-km/h (20-mph) urban access (AVV, 2001).