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Appendix E TRANSPORTATION AND CIRCULATION TECHNICAL DATA LEVEL OF SERVICE CONCEPT Level of Service (LOS) is a concept developed to quantify the degree of comfort (including such elements as travel time, number of stops, total amount of stopped delay, and impediments caused by other vehicles) afforded to drivers as they travel through an intersection or roadway segment. Six grades are used to denote the various LOS from A to F. A. Signalized Intersections The six LOS grades are described qualitatively for signalized intersections in Table E-1. Additionally,Table E-2 identifies the relationship between level of service and average stopped delay per vehicle. Using this definition, a "D" LOS is generally considered to represent the minimum acceptable design standard. B. Unsignalized Intersections The calculation of LOS at an unsignalized intersection requires a different approach. The 1985 Highway Capacity Manual includes a methodologgyy for calculating the LOS at two-way stop-controIled intersections. For these unsignalized intersections,LOS is defined differently than for signalized intersections in that it is based upon the concept of"Reserve Capacity" (i.e.,that portion of available hourly capacity that is not used). A qualitative description of the various service levels associated with an unsignalized intersection is presented in Table -3. A uantitative definition of LOS for a unsignalized intersection is presented in Table . The reserve capacity concept applies only to an individual traffic movement or to shared lane movements. Once the capacity of all the individual movements has been calculated and their LOS and expected delays determined, an overall evaluation of the intersection can be made. Normally,the movement having the worst LOS defines the overall evaluation,but this may be tempered by engineering judgment. An"E"LOS is generally considered to represent the minimum acceptable design standard. It should be noted that the Draft Traffic Impact Study guidelines has differing levels of LOS for the minimum acceptable design standard for all intersections (both signalized and unsignalized) depending upon the relative contribution of project traffic to the signal warrant. Past experience with the unsignalized analysis procedure indicates this methodology is very conservative in that it tends to overestimate the magnitude of any potential for that might exist. This is especially true for minor street left turn movements. For example, thelighway Capacity Manual methodology does not take into account the effects of vehicle flow platoons that result from upstream signalization. Vehicles traveling in platoons tend to create greater gaps in the traffic flow that sometimes provide additional capacity for the side closest to the signal. Therefore, the results of any unsignalized intersection analysis should be reviewed with this thought in mind. Generally,LOS E for the minor street left turn movement is considered to be acceptable for an unsignalized intersection, although it also indicates that the need for signalization should be investigated. E-1