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SECTION 2 Speed Considerations 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

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6 Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections 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 Exhibit 2-1. Roadway segment and intersection speed relationships.