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16 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections 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.

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Selecting an Appropriate Treatment 17 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

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18 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections 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 * Defined by Geometric and Operational Influences Exhibit 3-1. Roadway segment and intersection speed relationships.

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Selecting an Appropriate Treatment 19 Target Perception Deceleration Total Approach Reaction Distance (feet) Transition Speed (Va) Distance Distance (mph) (feet) (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. 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.