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Suggested Citation:"Section 2- Speed Considerations." 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 2- Speed Considerations." 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 2- Speed Considerations." 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 2- Speed Considerations." 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 2- Speed Considerations." 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 2- Speed Considerations." 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 2- Speed Considerations." 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 2- Speed Considerations." 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 2- Speed Considerations." 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 2- Speed Considerations." 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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2.1 Overview Although much is often assumed regarding the role of speed at intersections, little data exist that isolate the effects of speed on overall intersection performance (safety, operations, and ability to serve all modes). Speed is a product of many roadway and intersection features and, in turn, speed affects the performance of roadway facilities and the quality of adjacent environments. Speed reduction does not necessarily guarantee safety, nor does it guarantee operational or environmental benefits. Rather, the specific conditions of an intersection must be considered to determine what speeds are desirable for that particular location and environment. Speed may be deemed “excessive” when drivers do not have sufficient time to react to and safely navigate around interruptions in the flow of traffic or adapt their operations to the cur- rent conditions at an intersection. Excessive speeds generally result when environmental and operational elements are incompatible, sending motorists a mixed message about appropriate behavior. Excessive speed may result when a driver misinterprets the tasks needed to operate safely. In some cases, excessive speed may be a deliberate result of driver attitude, risk assessment, and behavior. The conditions at an intersection may require an operating speed that is slower than required by the conditions of the adjacent roadway segments. Defining the intersection influence area and the transition area is necessary to identify the area within which speed reduc- tion treatments are needed. This section focuses on the role of speed in an intersection environment and discusses the ways in which speed affects intersection performance and the adjacent environment. It details ways in which speed is affected by roadway design and elements of the adjacent environment. This sec- tion also highlights some physical conditions and user characteristics that may make an inter- section particularly sensitive to speed. The considerations presented in this section may help practitioners understand concerns related to speed. The following discussion outlines an approach to consider the operational qual- ities of a specific location and factors a user may wish to consider when investigating intersec- tion operations and design. 2.2 Intersection/Segment Relationship Defining the influence area of an intersection is fundamentally necessary to differentiate between reducing speeds on the segment rather than the intersection proper. The Guidelines focus on speed reduction treatments within an intersection’s geometric and operational influ- 5 S E C T I O N 2 Speed Considerations

ence areas and do not specifically address speed reduction in roadway segments. This report defines an intersection by geometric and operational influence area as follows: • Geometric—The location where the typical section of the roadway segment is modified to cre- ate the intersection features. These modifications include tapers for adding or dropping lanes approaching and departing from the intersection. • Operational—The area that is influenced by traffic operations, including queuing, lane chang- ing, merging, and vehicle acceleration/deceleration capabilities. This operational influence area could be independent of the geometric influence area and can change by time of day, sea- son, or other conditions. Speed transition needs should be considered between a roadway segment and the intersection influence area to allow drivers the opportunity to react to changing conditions and adjust their speed accordingly. This could potentially include a change in the roadway cross section (i.e., adding curbs and landscaping or via a “gateway” treatment) or simply providing adequate sight distance from the upstream segment to the intersection’s geometric or operational influence area. The length needed for the transition area will vary depending on the total desired speed reduction and the operating speeds in upstream segments. Exhibit 2-1 schematically depicts the roadway segment and intersection speed relationships. In some cases, the design speeds of the adjacent roadway segments are appropriate for an intersection. In other cases, the intersection characteristics and driver workload vary and a reduced speed may be desirable. The need for speed reduction at intersections can be considered in the following general conditions: • The posted speed of the segment is higher than the desired speed of the intersection approach (e.g., the intersection approach is stop controlled, or a transition from a rural to a more urban- ized environment occurs at the intersection). • The posted speed of the segment is the same as the desired speed of the intersection approach; however, drivers exceed the posted speeds. • The posted and operating speeds at the segment and intersection are reasonable. However, potential conflicts at the intersection (e.g., diverging or merging maneuvers, crossing traffic, or queues) require drivers to be especially alert to the need to respond to these potential conflicts. Stop-controlled intersection approaches will fall under the first condition, while uncontrolled and yield-controlled approaches may fall under any of these conditions. A stop-controlled condition requires operations that are independent of the roadway design speed (i.e., a tangent intersection 6 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Exhibit 2-1. Roadway segment and intersection speed relationships.

approach on level grade with unlimited sight distance has an unlimited design speed regardless of the posted speed approaching the intersection). A yield condition may operate as a stop control dur- ing some time periods, depending on traffic flows. In other cases, a yield condition may be virtually free flow and the operating speeds for this movement may be dictated by the roadway approach geometry (i.e., a separate turn lane, turning roadway width, and turning roadway radius). 2.3 Designing for Appropriate Speeds In general terms, good roadway geometric design provides a sufficient level of mobility and land-use access for motorists, bicyclists, pedestrians, and transit while maintaining an appropri- ate degree of safety. Higher-speed roadways are typically provided in locations where travel time and mobility are priority needs. Speed is often used as a performance measure to evaluate the effectiveness of highway and street designs. High speeds generally are associated with long trips, and low speeds generally are associated with short trips or with facilities that have more frequent access. Posted speeds frequently correlate with these intended uses. High-speed facilities serve key network needs—it is not always appropriate to expect reduced speeds at intersections. Environmental and operational indicators should be in place to provide drivers with a con- sistent message about the potential for conflict so they are best able to select an appropriate speed. The goal is to provide geometric street designs that look and feel like the roadway’s intended purpose. Because drivers choose their speed based on what they see on the roadway ahead, calling a driver’s attention to roadway features that present a potential risk provides increased opportunities to avoid conflicts. Drivers who perceive potential risk can better adapt their driving behavior to roadway conditions. A facility’s design speed is a fundamental design criterion that affects three-dimensional road- way design parameters (plan, profile, and cross section). Aside from roundabouts, where entry speeds of about 25 mph are specifically attained through geometric design, there is no common intersection design speed. Intersection speed typically is assumed to be that of the roadway segment. Although designers generally seek speed and operational consistency, intersection operations (e.g., queuing, deceleration, turning vehicles) and/or geometry may create localized conditions that require reduced speeds. Drivers must perceive and comprehend a greater variety of situa- tions at an intersection than while driving through the high-speed roadway segment that pre- cedes the intersection. Intersection conditions should be considered independently from the adjacent roadway seg- ments. For new facilities, this means ensuring that an intersection’s operational and geometric elements are appropriately configured. The existing geometric and operational elements of an intersection should be assessed when considering appropriate actions for retrofit projects. Such a design philosophy and approach can produce geometric conditions that are more likely to result in operating speeds consistent with driver expectations and commensurate with the roadway’s function. This, of course, does not account for impaired or overly aggressive motorists or conditions such as adverse weather. Ideally, drivers should operate their vehicles in a manner appropriate for the conditions for as long as those conditions prevail. 2.4 Factors Affected by Speed High speeds serve key network functions. However, operating speeds inconsistent with the prevailing conditions may adversely affect environmental quality and safety, or require a larger- than-desirable facility size. Speed Considerations 7

The effects of speed are most pronounced at intersections where friction between competing movements is concentrated. This friction may manifest itself as decelerating or accelerating vehicles, queues, crossing traffic, or traffic yielding to crossing pedestrians. Any of these kinds of friction create the potential for conflicts. The ways in which speeds through intersections affect intersection operations, environmental quality, and safety are discussed below. 2.4.1 Facility Size Drivers’ perception–reaction times are essentially constant; therefore, higher speeds require drivers to understand their driving tasks farther in advance of intersections, compared to slower- speed environments. This means that the distance between driver decision points must increase as speeds increase. Higher-speed facilities require larger clear zones and flatter horizontal and vertical curves. In addition, fundamental dimensions for stopping-sight distance increase with speed, leading to flatter and more open roadways. Attaining appropriate sight distances affects horizontal and vertical alignments as well as such cross-sectional features as cuts, fills, and landscaping. For example, using values from Exhibit 9-58 from AASHTO’s Policy on Geometric Design of High- ways and Streets, known as the “Green Book,” (AASHTO, 2004) for the case of a right turn from a stop or a crossing maneuver, the intersection sight-distance value for 45 mph is 430 ft. This value increases to 575 ft when speeds are 60 mph. In urban environments, sight-distance needs can affect building setbacks, on-street parking locations, and other design elements such as the locations of street furniture and landscaping. The transition zones within which drivers are required to slow down as they approach an inter- section need to be longer when a greater change in speed is required. This affects the length of alignment tapers, bay tapers, and the deceleration components of turn-lane designs. 2.4.2 Quality and Comfort of the Roadway Environment The function of roadways and intersections must be balanced with the needs of adjacent land uses to both maintain environmental quality and to provide necessary mobility. High-speed intersections can create a barrier to the mobility of non-auto users crossing the facility. The increased noise levels and intense environments that result from proximity to motor vehicles can create discomfort for pedestrians and bicyclists who travel parallel to the facility. Adjacent neigh- borhoods and businesses may also experience adverse effects from high-speed traffic, such as tire and engine noise, and may benefit from buffer treatments. Intersections near schools, hospitals, or other concentrations of pedestrians—particularly people who are elderly, young, or have disabilities—may be particularly sensitive to high speeds. 2.4.3 Safety The relationship between speed and intersection safety is a critical concern for transportation professionals. Road safety is often divided into three constituent elements: exposure (increases with the number of conflicting movements), risk (increases with traffic volume), and conse- quence (increases with speed). Reducing speed at intersections has the potential to improve con- sequence, although it has little relationship to exposure or risk. There is a decisive relationship between speed and crash severity, but the relationships between speed and crash frequency are less clear. 8 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

The physical relationship between mass and energy explains that higher speeds and larger speed differentials create the potential for higher-severity crashes. As speeds increase, the energy from the mass of the vehicles increases. Studies of modern multilane roundabouts illustrate the relationship between speed and crash severity: total crash rates and frequency may stay the same after an inter- section is converted to a roundabout, but the slower speeds help reduce the severity of crashes. A variety of intersection traffic conditions create large speed differentials, increasing the potential for severe crashes. For example • An unbalanced distribution of traffic for a given number and arrangement of lanes could create high differentials in speeds between vehicles traveling in the same direction. Consider that a channelized right-turn lane may operate under near free-flow conditions while the adja- cent travel lanes experience queuing. • An intersection or approach with extensive queuing, where the back of the queue is a signifi- cant distance from the intersection proper, could cause speed differentials. In these condi- tions, vehicle queues in turn lanes may exceed the storage length provided, requiring vehicles to decelerate in the through travel lanes. • Uniform traffic congestion at an approach could lead to queues that extend outside an intersec- tion approach’s typical available sight distance. In these conditions, vehicle queues extending beyond the intersection’s geometric influence area may require that drivers decelerate in advance of visual cues of the impending intersection. No research was found that identified a relationship between crash frequency and mean or 85th- percentile speed. However, many studies have found that the likelihood of being involved in a crash increases with deviation from the mean speed of traffic on the facility. (Taylor and Foody, 1965; Hauer, 1971) If the facility and intersection design provide adequate sight distance and appropriate user expectancy of potential conflicts, there should be adequate space and time to react to, and avoid, crashes. If, however, speeds are excessive for the facility and intersection design, there will be insufficient space and time to avoid crashes, and a high crash frequency may result. High-speed travel may affect crash avoidance because faster moving vehicles travel farther than slow moving vehicles during the typical reaction time needed for a driver to avoid a poten- tial hazard. In addition, the greater a vehicle’s speed, the less time there is for other motorists, bicyclists, or pedestrians to react to, and avoid, that vehicle. Thus, although there is no research to support the common assumption that reducing speeds will reduce crash frequency, reducing speed variation may achieve this. The relationships between intersection speed and safety are complex, and it cannot be assumed that reduced speeds will result in a safety improvement. 2.4.4 Traffic Operations Traffic operations refers to a roadway’s performance and is typically measured in terms of capacity, travel time, delay, number of stops, and queuing. Although vehicle speed directly cor- relates to the motorists’ perceived level of service along an arterial or highway segment, it does not have a significant effect on the traffic operational performance of an individual intersection. Furthermore, vehicle speed through the influence area of an intersection has little effect on over- all travel time. 2.4.4.1 Capacity Vehicle speed is not a primary determinant of intersection capacity—the number of vehicles that the intersection can process in a given time period. For the case of a minor street left-turn movement from a two-way, stop-controlled intersection, research performed as part of NCHRP Speed Considerations 9

Project 3-46, and documented in Capacity and Level of Service at Unsignalized Intersections, (TRB, 1996) found that speed did not have a significant effect on a driver’s critical gap, which corre- sponds directly to the capacity of the stop-controlled movement. For a signalized approach, the capacity of an intersection is a function of the saturation flow rate of the approaching lanes. Saturation flow rate is defined as the flow rate at which previously queued vehicles can traverse an intersection approach under prevailing conditions. As previously queued vehicles are starting from a stopped position, a vehicle’s speed through an intersection is not relevant in the calculation. 2.4.4.2 Travel Time Higher overall travel speeds along an arterial result in lower travel time and an improved level of service. Vehicle speed within the influence area of intersections does not generally have a sig- nificant influence on overall travel time. For example, given an intersection with a total influ- ence area of 1,000 ft, the travel-time difference between a vehicle traveling at 50 mph versus a vehicle traveling at 30 mph through the intersection is less than 10 seconds, assuming an uncon- trolled approach or a “green light” without interfering queues. 2.5 Factors that Affect Speed A driver’s selection of a safe speed and path is determined by his or her judgment, estimates, and predictions based on highway characteristics, traffic, and traffic control devices. (Lerner, 2002) Roadway design elements, environment, traffic type, and other factors help drivers deter- mine an appropriate speed. Some elements affect traffic flow directly; others can influence driver behavior by contributing to the visual complexity (or simplicity) of the roadway edge. The design and characteristics of an intersection proper affect speed at the intersection as do the design and characteristics of the roadway facility and adjacent segments. This section presents a variety of human, vehicle, and roadway characteristics that affect drivers’ speed. 2.5.1 Roadway Facility Design and Characteristics Intersections are often relatively infrequent occurrences on high-speed facilities and drivers may expect that they can operate at a consistent speed. Without clear indications of the need to reduce speed and without adequate transition distance within which to do so, drivers will navi- gate the intersection area at speeds they deem appropriate for the adjacent roadway segments. The chosen speed may or may not be appropriate for the actual conditions at the intersection. The characteristics of the roadway segment prior to an intersection affect speeds at the inter- section. Exhibit 2-2 provides a list of roadway facility factors that may affect speeds on intersec- tion approaches. These features may influence driver behavior or vehicle operations and result in speed changes. Many of these relationships are derived from relationships documented in NCHRP Report 504: Design Speed, Operating Speed, and Posted Speed Practices. (Fitzpatrick et al., 2003) 2.5.2 Speed Adaptation Drivers often underestimate their speeds, particularly in the medium- and high-speed ranges. Thus, excessive speed is not always a conscious decision. In some cases, excessive speed can be attributed to speed adaptation. The speed adaptation hypothesis states that the perceived speed of one’s vehicle will be lower than the actual speed if the driver has recently operated the vehicle at a higher speed. 10 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections

Speed adaptation may contribute to excessive speeds in transition areas between rural and built environments, or between access controlled or other high-speed facilities and street envi- ronments that have driveways, multiple intersections, and non-motorized users. Drivers who have adapted to higher speeds may not appreciate the need to slow down at intersections. These drivers may have attained a feeling of comfort or safety that may not be appropriate for the potentially changing conditions at a high-speed intersection. 2.5.3 Intersection Design and Characteristics The physical characteristics of the intersection proper affect speed, as do the changing condi- tions at the intersection (i.e., lighting and congestion patterns). Exhibit 2-3 summarizes many intersection characteristics that may influence drivers’ speed choice. Exhibit 2-3 originates from relationships documented in NCHRP Report 504: Design Speed, Operating Speed, and Posted Speed Practices. (Fitzpatrick et al., 2003) Speed Considerations 11 Intersection Variable Potential Relationship to Speed Facility Type Speeds tend to be higher on higher-order facilities. Speeds tend to be slightly lower when a raised median or no median is provided than when a depressed median or a two-way, left-turn lane is present. Limited access, low signal density, unimpeded visibility, and viaducts may promote high speeds. Roadway Wide shoulders, medians, and overall Characteristics pavement widths are associated with higher speeds. Lane widths, horizontal/vertical geometry, sight distance, curbs, and bike lanes may influence measured speeds and desired speeds. Conflicts and As the distance between points of friction Friction (driveways, intersections, pedestrian crossings, lane drops) increases, speeds increase to a point and then plateau. Posted Speed Posted speed and 85th-percentile speed increase or decrease together. Roadside Higher speeds occur in rural and undeveloped areas Environment compared to urban or developed areas. Lower speeds occur in areas with higher levels of pedestrian activity. Pavement Type Poor, cracked, or uneven pavement and joint and Condition details may slow travel speeds, while smooth pavement may allow faster speeds. The absence of centerline or edge line markings is associated with lower speeds. Transition The intersection location in relation to the roadway segment (tangential, curvilinear, flat, mountainous) may influence measured and desired speeds. Exhibit 2-2. Roadway facility characteristics that may affect intersection speed.

2.5.4 Drivers and Vehicles Different types of drivers will choose different speeds at intersections. Commuters and other familiar drivers may tend to drive faster than infrequent users. Driver age and attitude also influ- ence speed through intersections. Additionally, transit vehicles, heavy vehicles, and recreational vehicles may have especially slow turning speeds, requiring them to travel slower than general traffic to maneuver through an intersection. 2.5.5 Weather Conditions Weather conditions may affect driver behavior and speeds. Drivers may use caution and drive slower during snow, ice, rain, fog, or dust than they do in clear and sunny conditions. 2.6 Conditions Potentially Sensitive to Speed This section identifies a variety of conditions, elements, and data that may indicate that an intersection is particularly sensitive to speed. In some cases, these are contextual considerations 12 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections Intersection Variable Potential Relationship to Speed Traffic A signalized intersection, a stop-controlled Control/Approach Type intersection, and a yield-controlled intersection require different driver tasks and operating speeds. Wayfinding Complex intersection maneuvers tend to reduce speeds, especially for unfamiliar drivers. Visual Complexity Roadside development, pedestrians, bicyclists, sig- nage, and environmental elements that interest driv- ers may affect speed. Roadside Impedances Roadside parking, bus stops, and vehicle loading zones may interrupt traffic flow near intersections and effect speed reductions. Lane Drops Drivers may slow down to make lane changes. Increased lane densities may also reduce speeds. Merging Through drivers may need to slow down to create gaps for vehicles entering the traffic stream. Sight Distance Drivers may travel at higher speeds through intersec- tions without sight-distance constraints. Sight-dis- tance restrictions can induce a small reduction in speeds, although only for the faster vehicles. Lighting Inadequate lighting may not allow drivers to per- ceive and react in advance of an intersection. Traffic Conditions Congestion, queuing, directional distribution, and low volumes influence speeds through intersections. Exhibit 2-3. Intersection characteristics that may affect intersection speed.

related to the intersection configuration, location, or environment; in other cases, the sensitivity might be attributed to specific users or user characteristics at that location. Field conditions may also provide insights into an intersection’s sensitivity to speed. This might include observing user behavior or evaluating traffic and safety data. 2.6.1 Common Conditions There are a variety of conditions that may be associated with a heightened sensitivity to speed. Many of these are related to the characteristics that affect speed identified in Exhibit 2-2. If driv- ers are alert to the characteristics of an intersection that make lower speeds desirable, they may slow down. If, however, they are not alert to these characteristics it can create a condition that is sensitive to speed. Examples include the following: • Intersections that are difficult to detect—horizontal or vertical curvature of an intersection approach may make it difficult to detect. • Intersections within a corridor with a variety of changing contexts (land uses, design philosophies)—drivers need to be warned of the increased need to respond to pedestrians, buses, driveway traffic, or other interruptions in traffic flow. • Intersections that link high-order and low-order roadway segments—gateways between rural and urban areas. • Intersections with complex geometry or irregular route continuity—requiring a high driver workload to comprehend or navigate through the intersection. • Intersections proximate to concentrations of sensitive or high-risk populations—children, elderly, or disabled. • An approach with limited sight distance—roadside obstacles, sunlight at certain times of day. • Intersections with high-speed differentials or limited acceptable gaps at certain times of day— long queues for one or more approach lanes. 2.6.2 Observed Field Conditions Field observations of an intersection’s operating conditions may provide an opportunity to identify intersections sensitive to speed. 2.6.2.1 Crash Avoidance Patterns Skid marks on a roadway are often an indicator that drivers do not have sufficient time to react to interruptions in the flow of traffic. The direction and location of skid marks may be useful clues for deciphering operating conditions. Skid marks may indicate that drivers are traveling faster than their ability to perceive and react to an intersection condition (i.e., a single, stopped vehicle turning left, a vehicle that is accelerating or decelerating, or the back of a queue that extends beyond the perceived area of the intersection). 2.6.2.2 Driver Behavior Undesirable driver behavior, such as lack of compliance with signals or stop signs, may indi- cate that excessive speeds approaching an intersection do not give drivers enough time to see and react to traffic control. The speeds may be the result of adaptation from the prior segment with drivers unaware of their speed and required stopping distances. Desired speeds might be attained if drivers had assistance in transitioning from one segment to the other. 2.6.2.3 Congestion Patterns Congestion patterns can increase the intersection influence area, creating sensitivity to speed. Congestion patterns that create high-speed differentials (such as those listed in Section 2.4.3), that are associated with increased pedestrian or transit use, or that reduce the availability of acceptable gaps for side street traffic, may indicate a particular sensitivity to speed. Speed Considerations 13

2.6.2.4 Speed Data Speed data can provide insights about effective roadway conditions. For example, the meas- ured 85th-percentile speed, mean speed, and speed variance can be compared to the implied design speed based on the “Green Book” (AASHTO, 2004) intersection-sight-distance and stopping-sight-distance criteria. Comparing measured speeds to the posted speed could help identify differences between desired operations (as indicated by a posted speed) and actual conditions. 2.6.2.5 Crash Data A high crash rate at an intersection may indicate any number of operational or geometric issues and does not, in isolation, indicate excessive speeds. A closer analysis of the crash types, locations, time of day, weather conditions, and other factors is useful for understanding the role of speed, if any, in the crashes. Crash patterns that may be associated with excessive speeds include the following: • Frequent rear-end crashes—drivers not anticipating the location of the back of a queue, • Frequent run-off-road crashes—drivers avoiding conflicts in the roadway proper, or • Angle crashes—drivers accepting gaps that are too small. A close analysis of the crash data is necessary to determine if these patterns could also be attrib- uted to other factors such as driver inattention or impairment, geometric conditions (alignment, sight distance), or other conditions that do not meet a driver’s expectations. 2.7 Summary Reduced speeds do not guarantee safety, nor do they guarantee operational or environ- mental benefits at a high-speed intersection. Speed is a product of many features, including the adjacent roadway segment, the user and vehicle type, and the general environmental con- text. Intersection approach speeds can affect a facility’s safety and performance. Speed, when considered as a design criterion or consideration, can affect the roadway design while also influencing the environmental context. Understanding how speed affects intersection condi- tions and how those conditions affect speed is considered first to evaluate and select an appro- priate speed reduction treatment. 14 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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