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10 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections 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. 18.104.22.168 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.
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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. 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)