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Speed Considerations 7 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.
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8 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections 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' perceptionreaction 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.
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Speed Considerations 9 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. 22.214.171.124 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