National Academies Press: OpenBook

Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (2008)

Chapter: Section 3 - Selecting an Appropriate Treatment

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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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Suggested Citation:"Section 3 - Selecting an Appropriate Treatment." National Academies of Sciences, Engineering, and Medicine. 2008. Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14162.
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3.1 Introduction There are a variety of treatments with the potential to reduce vehicle speeds at high-speed intersections. This section provides information about these treatments, including their applicability, cost, secondary impacts, implementation considerations, and potential to effec- tively reduce speeds and increase safety. This information was compiled from a variety of national and international sources. However, there are very little data that clearly quantify how effectively these treatments reduce speeds at high-speed intersections. Furthermore, there are relatively little data on the safety performance of potential treatments and, when information is provided, there is no direct correlation to the change in safety caused by reduced speeds. Quantifying the safety effects of speed reduction treatments at high-speed intersections will require conditions to be monitored for a number of years after the treatments are implemented. The information provided in this section highlights the considerations that will help deter- mine which treatments may be appropriate at a specific location. In many cases, one or more treatments may be appropriate for a given intersection. To determine the most appropriate treat- ment, the information provided in this section must be balanced with local practices and engi- neering judgment related to the specific situation. The treatments covered by the Guidelines include those commonly used in the United States, those currently being researched, and those for which there was an expressed interest by state highway agencies. National use and interest in the treatments was determined through a national survey of state highway agencies conducted as part of the research efforts for NCHRP Project 3-74. More information about the survey and the responses can be found in NCHRP Web-Only Document 124. Some of the treatments included in the Guidelines have had limited or no documented appli- cation at high-speed intersections; however, their functions indicate they have potential to be applied at certain high-speed locations. For example, speed tables are generally used in low-speed residential environments, but they may be able to be applied at the stop-controlled approaches of high-speed intersections to reinforce the need to decelerate in advance of the intersection proper. Additionally, many of the treatments included in the Guidelines can be applied in both high- speed and low-speed environments. Identifying a treatment as a “high-speed” treatment does not imply it is inappropriate for low-speed applications. 15 S E C T I O N 3 Selecting an Appropriate Treatment

3.2 Determining the Need for a Treatment Speed reduction treatments are appropriate when an engineering study indicates the need for reduced speed. The Manual on Uniform Traffic Control Devices (MUTCD) (FHWA, 2003) lists the following factors that may be considered when establishing speed limits: • Free-flowing traffic’s 85th-percentile speed; • Road characteristics (shoulder condition, grade, alignment, and sight distance); • Pace speed; • Roadside development and environment; • Parking practices and pedestrian activity; and • Reported crash experience for at least a 12-month period. Logic indicates that similar elements should be considered when determining the need for speed reduction treatments. It is not advisable to install treatments based on requests from local officials or residents when an engineering study has not taken place. In some cases, treatments might be applied to define the location where deceleration should begin to attain the desired speeds in the intersection area. In other cases, the visible presence of treatments at the intersection proper (i.e., roundabouts, splitter islands) may be sufficient to cause the desired speed reduction in advance of the intersection. A treatment may not signifi- cantly reduce speeds; however, that does not necessarily mean the treatment is ineffective. There may be safety benefits to alerting drivers to the changing roadway conditions as they travel from the roadway segment to the intersection influence area. 3.3 Treatment Selection Process Specific treatments are not necessarily appropriate in all circumstances and conditions. A treatment applied at one location on a facility may not be appropriate at a different location on the same facility. The unique characteristics of each intersection and the speed issues that exist must be assessed during the selection process. Many treatments are appropriate to use in a vari- ety of conditions, but it is likely that their effectiveness will vary considerably depending on the conditions of the specific application. This section describes a process to select appropriate speed reduction treatments for a particular condition. The steps of the treatment selection process are • Intersection pre-screening, • Treatment screening, and • Treatment implementation. Appendix A provides a treatment implementation process framework to help the user assess an intersection and potential treatments. Appendix B provides several case studies that detail treatment selection and design. 3.3.1 Intersection Pre-Screening The pre-screening process involves identifying an intersection that may benefit from a speed reduction treatment and then assessing the data for that intersection to make decisions about which treatments may be appropriate and most effective at a particular candidate site. Gathering data (i.e., crash history, speed study data, and aerial photos) can be helpful in understanding the site context, including the relationships between the roadway segment and intersection influence area, as well as the influence of the overall facility geometry and environment. Understanding these aspects of the intersection and segment can then help determine if speed is the contributing factor for the problems experienced at a particular location. 16 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

3.3.1.1 Identify Intersection as Needing Attention The first step in intersection pre-screening is identifying the need for further study of speed issues at the intersection. This determination may be based on reports of high speeds, high crash frequency and/or severity, recurring crash types, and/or near misses. 3.3.1.2 Gather Intersection Information The next step is to gather information about the intersection including intersection features, crash history, speed data, traffic volumes, traffic composition, aerial photographs, and site observations. 3.3.1.3 Assess Data The final step of pre-screening is assessing the data to determine whether speed is a primary problem. Appropriate questions to answer are as follows: • What is the primary problem? • Is speed a contributing factor or should other actions be considered? • What can be learned anecdotally from agency staff? 3.3.2 Treatment Screening After the pre-screening process is completed to verify that speed may be a primary issue, the treatment screening process is used to eliminate candidates with “fatal flaws” and identify prom- ising treatments. 3.3.2.1 Identify Fatal Flaws Fatal-flaw screening may eliminate treatments based on cost, agency policies, or existing inter- section characteristics. Examples of fatal-flaw screening include • Cost—funding issues may prevent installing a dynamic warning sign or approach curvature. • Time to implement—some treatments take longer to install than others, and some can only be installed during certain weather conditions. • Noise considerations—rumble strips may be undesirable in residential areas. • Right of way—approach curvature may require right of way. • Energy/power source—dynamic warning signs require a power source. • Land use and environment—existing driveway locations may prohibit some types of treatments. • Policy—some jurisdictions may have policies that prohibit some treatments. • Novelty—depending on the user (commuter, recreational), some treatments may be more prone to having limited long-term effects. 3.3.2.2 Evaluate Potential Treatments After the list of treatments has been narrowed through the fatal-flaw analysis, the next step is to gain a better understanding of each remaining potential treatment and determine the objec- tives for the treatment. Appropriate questions to answer are as follows: • What is the target speed? • Where speeds should be reduced, how much should the speeds be reduced? • What information is available about each treatment? • Has there been any past research conducted on that particular treatment? Was the treatment effective? Were there any side effects of the treatment? The Guidelines do not address the first two questions regarding target speed and the desired speed reduction. These must be determined based on the specific conditions and context of the intersection. Later in this document, Exhibits 3-3 and 3-4 and Section 4 provide information to Selecting an Appropriate Treatment 17

answer the last two questions regarding available treatment information, treatment research, and effects. This information should be supplemented with local experience and professional judg- ment to select one or more treatments for implementation. 3.3.3 Treatment Implementation The basic goal of the treatments is to achieve a target approach speed at or before drivers reach the intersection influence area. To achieve this, it is useful to determine the boundaries of the intersection influence area, the target approach speed, and the length of the transition area. Exhibit 3-1 shows the key elements used in treatment design. 3.3.3.1 Target Approach Speed The target approach speed generally is selected so that drivers can operate safely and with- out an adverse affect on non-motorized users within the influence area. For example, if the geometry of an intersection approach has limited sight distance, the target speed may be selected such that vehicles traveling at that speed will have sufficient stopping sight distance as they approach the intersection. Alternatively, if there is a school in one quadrant of an inter- section, the target speed may be selected such that the noise and perceived risk are acceptable. In either case, the target speed should be reached in advance of the intersection influence area to ensure acceptable conditions within the intersection influence area. The location of the tar- get speed may be affected by the intersection’s geometric and operational influence area. For example, a recurring long queue on one approach may shift the target speed location upstream on the roadway segment to provide sufficient distance for drivers to comfortably decelerate to the back of the queue. 3.3.3.2 Intersection Influence Area As described in Section 2.2, the intersection influence area is determined by the geometric and operational influences. Determining the operational influence area generally involves iden- tifying the location of a potential conflict (i.e., a crossing maneuver, yield point, or back of queue) and calculating the stopping sight distance needed in advance of that location. For a stop-controlled approach, a comfortable deceleration rate should be assumed. For other approach types, it is appropriate to assume a rapid deceleration rate. Exhibit 3-2 shows the 18 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections * Defined by Geometric and Operational Influences Exhibit 3-1. Roadway segment and intersection speed relationships.

required transition that should be assumed in determining the operational influence area for several conditions. 3.3.3.3 Transition Area Upstream of the intersection and within the roadway segment, a transition area may be required for drivers to adjust their speeds to the target speed. The size of the transition area will vary based on segment speed and the amount of speed reduction required. The length of the transition area should support comfortable deceleration rates. Exhibit 3-3 shows appropriate transition distances for several roadway segment and target speed conditions. 3.3.3.4 Treatment Area To achieve this pattern of speed reduction, treatments should be designed to take effect at the beginning of the transition area. Depending on the type of treatment, it may be appropriate to carry the treatment through the intersection, or, as in the case of a dynamic warning sign, the treatment may be placed in one discrete location. In either case, the treatment should be per- ceived by motorists no later than the beginning of the transition area to ensure that adequate space and time are provided for the desired speed reduction. Treatment design should reinforce existing intersection features including signs, lighting, pavement markings, lane drops or added lanes, medians and splitter islands, horizontal and vertical curves, and any other features that provide visual cues of the impending intersection. Typical layouts for treatment designs can provide guidance. However, modifications based on specific conditions will generally provide the best results. Selecting an Appropriate Treatment 19 Speed (Va) (mph) Distance (feet) Distance Target Approach Perception Reaction Deceleration Distance (feet) Total Transition (feet) Uncontrolled or Yield Intersection Approach 50 185 530 715 45 165 435 600 40 150 320 470 35 130 280 410 30 110 235 345 Stop- or Signal-Controlled Intersection Approach 50 185 240 425 45 165 195 360 40 150 155 305 35 130 120 250 30 110 90 200 20 75 40 115 Notes: Va is the average speed of the roadway segment. Perception and reaction time is assumed at 2.5 seconds. Deceleration distances for uncontrolled or yield intersection approaches are interpreted from Chapter 10: Grade Separations and Interchanges, Exhibit 10-73. (AASHTO, 2004, p. 851) Deceleration distances for stop- or signal-controlled intersection approaches are from Exhibit 3-1. (AASHTO, 2004, p. 112) Exhibit 3-2. Intersection area: operational influence distance from conflict or stop.

3.4 Combining Treatments Intuition suggests that combining treatments will increase the potential to reduce speed. Although there is little research that determines this potential, it is expected that combining treatments will have a benefit up to a point, after which, no more speed reduction will occur. This is because drivers will operate at a speed at which they feel comfortable or safe. Below this speed, the cumulative application of treatments becomes ineffective. Additional treatments can provide benefits by reinforcing the need to be prepared to slow down, even if additional speed reduction is not observed. An example of combining treatments is a low-cost concept for two-way, stop-controlled intersections on high-speed, two-lane rural highways as studied by FHWA and documented in Low-Cost Intersection Treatments on High-Speed Rural Roads. (FHWA, 2006) As shown in Exhibit 3-4, the concept incorporates a variety of individual treatments, including lane nar- rowing, splitter islands on each approach, and lateral pavement markings on each side of the traveled way. The objectives of this FHWA study were to identify, promote, and evaluate low-cost concepts to reduce speeds at intersections. The research team investigated two concepts that combined various roadway treatments. The first concept was to reduce the lane width, add rumble strips, and add pavement markings on the major road. The second concept was to install a mountable splitter island with stop signs on the minor road approaches. The treatment concepts were implemented in multiple locations in Pennsylvania, New Mexico, and Illinois. The results from this study showed statistically significant speed reduction at all sites. The combination of treatments reduced all vehicle speeds by an average of 3 mph, and reduced the 85th-percentile speeds by 4 mph. In addition, testing results for trucks revealed an average speed reduction of 5 mph and a reduction in the 85th-percentile speeds of 4 to 5 mph. (FHWA, 2006) The team collected crash data for five years before deployment and will collect data for two years after deployment to determine whether these treatments yield quantifiable results. The team also plans to analyze crash data at the various sites at the project’s end in June 2009 because 20 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Mid-Block Speed (Vs) Target Approach Speed (Va) (mph) Perception Reaction Distance (feet) Deceleration Distance (feet) Total Transition Distance (feet) 60 45 220 390 610 60 20 220 620 840 55 45 200 280 480 55 20 200 500 700 50 45 185 240 425 50 20 185 480 665 45 35 165 225 390 45 20 165 350 515 Notes: Vs is the average speed of the segment. Va is the target approach speed. Perception and reaction time is assumed at 2.5 seconds. Deceleration distances are interpreted from Chapter 10: Grade Separations and Interchanges. (AASHTO, 2004) Exhibit 3-3. Transition distance between segment and intersection area.

several years of data are needed to determine the potential effects the treatments had on road- way safety. (FHWA, 2006) 3.5 Treatment Considerations This section provides information to aid in screening treatments and to identify one or more treatments that may be most appropriate for a particular condition. Exhibits 3-5 and 3-6 pro- vide a summary of the considerations for each treatment. Treatments for which specific testing has been conducted are shown in the shaded sections. As described in the following sections, Exhibit 3-5 summarizes typical uses of the treatments, indicates how they may be applied at high-speed intersections, indicates whether research was found to document their effectiveness, provides order-of-magnitude cost relationships, and indicates the treatments’ experimental status with respect to the MUTCD. Exhibit 3-6 provides the documented treatment applications, their effectiveness, and noteworthy considerations. Section 4 provides supplemental information, diagrams, and photos for each treatment. The information provided in Section 4 represents the best available data based on the extensive lit- erature review and field testing conducted under NCHRP Project 3-74. Selecting an Appropriate Treatment 21 (Credit: FHWA, 2006) (a) Reduced lane width, added rumble strips, and added pavement markings on major road (Credit: FHWA, 2006) (b) Installed mountable splitter island with stop signs on minor road approaches Exhibit 3-4. FHWA low-cost treatment concepts.

22 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Treatment Documented Uses Potential High-Speed Intersection Applications Dynamic Warning Signs Horizontal curves Work zones Rural, unsignalized intersections Use caution in high driver-workload contexts Transverse Pavement Markings Traffic circle and stop-controlled intersection approaches Horizontal curves, bridges, freeway off-ramps Work zones History of high-speed crashes Transverse Rumble Strips Approaches to intersections Approaches to toll plazas Use caution in noise-sensitive contexts and with motorcycle traffic Longitudinal Rumble Strips Rural highways Consider impacts to bicycle traffic Wider Longitudinal Pavement Markings Crash history involving curves, hills, roadway cross section Work zones Run-off-road crashes Use caution as increased visibility may lead to increased speeds Roundabouts Rural highways Transition areas Rural Highways Gateways Use caution with steep grades, unusual geometry, constrained right-of-way Approach Curvature Roundabout approaches Stop- and yield-controlled approaches Use caution with run-off-road crash history, grades Splitter Islands Roundabout and stop-controlled approaches Minor approaches of T-intersections and two-way, stop-controlled intersections Speed Tables/ Plateau Local and collector streets Plateaus installed on facilities with 35 to 50 mph speeds in New Zealand Stop-controlled approaches Reduced Lane Width Work zones Low-speed urban and residential locations Rural two-lane highways Uncontrolled approaches Visible Shoulder Treatments Rural two-lane highways Wide shoulders Run-off-road crashes Roadside Design Features (Including Gateways and Landscaping) Transition areas Adjacent to roadway or intersection beyond clear zone and line of sight Transition areas Consider sight distance at driveways Note: Shaded rows indicate treatments tested through NCHRP Project 3-74. Exhibit 3-5. Treatment application summaries matrix.

Selecting an Appropriate Treatment 23 Treatment Function Documented Speed Reduction Documented Safety Improvement Cost Experimental Status Dynamic Warning Signs Encourage deceleration Reduce comfortable approach speed Yes Yes $$ No Transverse Pavement Markings Improve intersection visibility Alert drivers to upcoming intersection Yes Yes $ Yes Transverse Rumble Strips Encourage deceleration Reduce comfortable approach speed Yes No $$ No Longitudinal Rumble Strips Reduce comfortable approach speed No data Yes* $$ No Wider Longitudinal Pavement Markings Increase visibility and demark the intersection influence area No data No data $ No Roundabouts Reduce speed through intersection Reduce conflicts Yes Yes $$$ No Approach Curvature Reduce comfortable approach speed Yes* Yes $$$ Yes Splitter Islands Improve intersection visibility Alert drivers to upcoming intersection No data Yes $$ Yes Speed Tables/Plateau Encourage deceleration Reduce comfortable approach speed Yes* Yes* $$ No Reduced Lane Width Reduce comfortable approach speed Yes* No $$ No Visible Shoulder Treatments Alert drivers to upcoming intersection Improve intersection visibility No data Yes $ No Roadside Design Features (Including Gateways and Landscaping) Reinforce transitioning roadway environment Improve intersection visibility Reduce comfortable approach speed Yes* Yes* Varies No Notes: Shaded rows indicate treatments tested through NCHRP Project 3-74. * Indicates data for low-speed intersections and/or roadway segments only. Exhibit 3-5. (Continued).

24 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Treatment Documented Applications Documented Speed Effects High-speed intersections 2 test sites (WA, TX) uncontrolled approaches Mean speeds reduced up to 2.8 mph Initial reduction of 3 to 4 mph at two TX sites, one site had sustainable results Dynamic Warning Signs Others Rural interstate work zones (SD, VA) Freeway ramps (MD, VA) School zones (TX) Curve approaches 1.4 to 4 mph mean speed reduction in work zones Reduction in the number of speeding vehicles in work zones: 20–25% for cars, 40% for trucks High-speed intersections 5 test sites (OR) Stop-controlled and uncontrolled approaches Up to 0.9 mph mean speed reduction Transverse Pavement Markings Others Horizontal curves Roundabout approaches Work zones Highway off-ramps Bridges 20-30% speed reduction on segments Peripheral lines almost as effective as full transverse lines High-speed intersections 3 test sites (TX) uncontrolled approaches 1.3 mph mean speed reduction at the perception-response time location Transverse Rumble Strips Others Some stop-controlled approaches Toll plazas, horizontal curves, and work zones More gradual deceleration Increased speed variation Not successful in significantly reducing approach speeds High-speed intersections No data No data Longitudinal Rumble Strips Others Shoulder or center of expressways, parkways, and rural highways No data High-speed intersections No data No data Wider Longitudinal Pavement Markings Others Work zones Vertical and horizontal curves Access-controlled highways Toll roads No data High-speed intersections Roundabouts Others Many applications on high-speed and low- speed facilities Roundabout type and design vary based on facility speed, access, volumes, vehicle composition, and other factors Geometry yields 13 to 17 mph 85 th - percentile entry speeds Note: Shaded rows indicate treatments tested through NCHRP Project 3-74. Exhibit 3-6. Treatment effectiveness summaries.

Selecting an Appropriate Treatment 25 Treatment Documented Safety Effects Key Considerations No data Dynamic Warning Signs Reduction in rollover crashes on freeway ramps Power supply Urban vs. rural applications Target speed Coordinate with other treatments closer to intersection No data Transverse Pavement Markings 50-65% crash reduction at roundabouts and high crash locations Driver familiarity Human factors considerations No data Transverse Rumble Strips Marginal safety benefits at best Noise impacts to adjacent land uses May make bicycling difficult Vibrations can startle drivers and bicyclists, causing quick and unsafe maneuvers Maintenance No data Longitudinal Rumble Strips Reductions in injury, sideswipe, and opposite direction crashes 46% reduction in shoulder encroachment Truck traffic, if narrowing lane width Vibrations can startle drivers and bicyclists, causing quick and unsafe maneuvers Maintenance No data Wider Longitudinal Pavement Markings No data Improved visibility, especially with older drivers Roundabouts All roundabout conversions: approximate 35% reduction in total crashes and 77% reduction in injury crashes Rural two-way stop conversions: approximate 72% reduction in total crashes and 87% reduction in injury crashes Operational performance Right-of-way needs Pedestrian and bicycle safety Gateway treatment Note: Shaded rows indicate treatments tested through NCHRP Project 3-74. Exhibit 3-6. (Continued).

3.5.1 Typical Uses Many of the treatments mentioned in Section 4 have not been documented as having been used at high-speed intersections; however, they may be effective for those applications. The typical uses listed for each treatment are provided to indicate applications for which the treatment is known to have been used. The documented effects are primarily based on these known applications. The lists of typical uses are helpful when considering driver expectations associated with a treatment. 3.5.2 High-Speed Intersection Applications The high-speed intersection applications provided in Section 4 describe high-speed intersec- tion conditions for which a treatment may be well suited. This section also describes conditions for which the treatment is not well suited. For example, dynamic warning signs may be appro- priate on approaches to rural unsignalized intersections, but may not be appropriate in contexts where there is already a high driver workload or in a complex visual environment where the dynamic warning signs may prove ineffective. 3.5.3 Function The function(s) of the treatments describe the role that each treatment plays in the intersec- tion environment to produce lower speeds. The treatments should be screened to ensure that those selected have functions that are suited to the specific speed problem(s) identified in the intersection assessment. For example, if speed adaptation is identified as the root of a speeding problem, it is likely that a treatment that reinforces an impression of changes in the character of the roadway environment sufficiently in advance of the target speed location will be most suc- cessful in reducing speeds. Similarly, if driver attentiveness is identified as the likely root of the problem, appropriate treatments might be those that alert drivers to the upcoming intersection. 3.5.4 Documented Effects There is limited documented research that identifies how much speed reduction (if any) may be expected from a given treatment. Most available documentation relates to segment speed reduction applications rather than intersection speed reduction. However, these data are included because it is logical to assume that, in many cases, similar effects may be achieved on high-speed intersection approaches. Data that specifically address speed reduction effectiveness on high-speed intersection approaches are available for the following treatments: dynamic warn- ing signs, transverse pavement markings, roundabouts, approach curvature, and rumble strips. Because the extent of these data is quite limited, the speed reduction actually realized will likely vary with each application. The information available related to each specific treatment’s effec- tiveness is provided as part of the detailed treatment descriptions. 3.5.5 Cost Cost is often a factor that limits treatment implementation. In general, treatments that do not involve changes to the paved roadway section have the lowest costs, while treatments that mod- ify or reconstruct the roadway section are the most costly. Secondary costs such as establishing a power source in a rural location could also increase implementation costs. 3.5.6 Experimental Status Identifying whether a treatment would be considered experimental—whether or not it is included in the MUTCD—helps the user determine if additional authorization may be needed before an experimental treatment can be installed. 26 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

3.5.7 Documented Applications, Speed Effects, and Safety Effects The columns for documented applications, speed effects, and safety effects in Exhibit 3-6 describe the tested applications for which the listed documented speed and safety effects were achieved. This information should be considered to determine the extent of research and data on a particular treatment and to identify similarities and differences in the tested applications that may produce similar or dissimilar effectiveness in a candidate site under consideration. 3.5.8 Key Considerations Many treatments are likely to have significant impacts on multimodal users, site-specific maintenance considerations, and/or other issues that should be considered as part of the screen- ing process. These secondary impacts may be limiting factors depending on the context for installation. For example, maintenance concerns related to snow removal will be significant in some environments and insignificant in others. Conditions in which particular treatments are not advised also are key considerations. 3.6 Treatment Evaluation Monitoring and evaluating the effectiveness of treatments can provide valuable data and feed- back for determining appropriate treatments for future applications. 3.6.1 Speed Monitoring Speed monitoring is the most direct way to evaluate the effectiveness of a speed reduction treatment. To evaluate speed effects, before-and-after data should be collected and compared. Driver acclimation effects can be expected with many of the treatments; therefore, the after-data collection should be targeted to identify both short- and long-term effectiveness. Speed moni- toring can also be used to evaluate how effective treatments are under particular weather or light- ing conditions. Data collection efforts should consider the driver behavior desired on the intersection approach, and should target data collection to determine to what extent the desired behavior has been achieved. In most cases, speed reduction is desired not only at the intersection proper, but on the approach as well. Collecting speed data at several points on an intersection approach will give the best picture of drivers’ deceleration curves (a speed profile) as they respond to the treat- ment(s) approaching the intersection. Appendix B identifies the speed data collection points for several case studies. Additional information about speed testing programs can be found in the testing results section of NCHRP Web-Only Document 124. 3.6.2 Safety Monitoring As stated repeatedly in these Guidelines, there are no data to support the use of speed as a sur- rogate for safety or vice versa. An improved safety record based on implementing one or more of these treatments does not necessarily indicate that speeds were reduced. Nevertheless, safety is a critical concern for transportation professionals, and monitoring the safety effects of these treatments may be desirable. Safety effects must be monitored and analyzed over a period of several years before any sig- nificant conclusions can be drawn. Crashes are infrequent events and extensive data are needed to evaluate improvements in safety performance. Selecting an Appropriate Treatment 27

3.6.3 Driver Perception, Satisfaction Surveys Surveying drivers about their perception of the treatment may provide insights into the effec- tiveness of a treatment. If drivers indicate that they were aware of the treatment, that the treat- ment increased their awareness of the intersection, or that it influenced them to slow down, these results may indicate that a treatment is effective. It is not advisable to implement treatments based on neighborhood or political complaints if these complaints are not supported by an engineering analysis that indicates a speeding prob- lem. However, the observations and perceptions of local residents and businesses can provide insight into the effectiveness of a treatment. 3.7 Summary Various treatments have the potential to reduce speeds at high-speed intersections. These include specific intersection treatments as well as design elements typically considered for road- way segments. Each intersection must be assessed to understand its unique characteristics and how it may be affected by speed. With this understanding, the user can consider which treatments are most likely to reduce speeds. The effectiveness of a particular treatment will vary considerably depend- ing on the conditions of the specific application. The potential treatments have a wide range of applicability, cost, secondary impacts, imple- mentation considerations, and potential effectiveness in reducing speeds and increasing safety. To quantify the safety effects of speed reduction treatments at high-speed intersections, conditions will need to be monitored for a number of years after the treatment is implemented. The next section provides more detail on each treatment type, including a discussion of treatment applications, considerations, and possible effectiveness. 28 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

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TRB's National Cooperative Highway Research Program (NCHRP) Report 613: Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections explores the effectiveness of geometric design features as well as signage and pavement markings to reduce vehicle speeds at high-speed intersections. A final report documenting the entire research effort is available online as NCHRP Web-Only Document 124.

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