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LEVEL OF SERVICE METHODOLOGY UNSIGNALIZED INTERSECTIONS According to the Highway Capacity Manual,capacity analysis for unsignalized intersections (with stop control only on minor street approaches)is based on the use of gaps in a major traffic stream by vehicles crossing or turning through that stream. At unsignalized intersections, "Stop" or"Yield" signs are used to assign the right-of-way to one street while controlling movements from the other street(s). This forces drivers on the controlled street,usually the "minor" street approach to the intersection,to judgmentally select gaps in the major street flow through which they can enter and turn into the intersection,or cross entirely through the intersection. The capacity analysis method used for unsignalized intersections generally assumes that major street traffic is not affected by minor street flows. This is generally valid for periods when traffic operations are smooth and uncongested;however,when long queues form on the minor street, major street traffic flows may experience some impedance from minor street traffic forcing its way into the major street traffic flow. Left turns from the major street are assumed to be affected by the opposing(or oncoming) major street flow. Minor street traffic movements are affected by all conflicting movements. In analyzing the ability of traffic to use gaps in the major street flows,it is generally recognized that certain"critical" movements(i.e., a traffic movement that requires crossing an opposing traffic flow) are more able to use these gaps than others. A right turn from the minor street represents the critical movement most able to use available gaps since the driver has to be concerned with gaps in only one direction of major street traffic. A left turn from the major street is the next critical movement most able to use available gaps,followed by through movements and left turns from the minor street. Drivers making left turns from the minor street onto a major two-way street must recognize and negotiate their way through gaps in two directions of major street flows. This concept is important because it highlights the frequent capacity shortages for vehicles seeking to make left turns from a minor street onto a major street. The key input data required to analyze unsignalized intersections include geometric characteristics and peak hour volumes. Geometric factors incorporated into the analysis include the number and use of lanes,channelization(i.e.,lane markings),roadway grades,curb radii and approach angles,and sight distances. Traffic volumes are converted to passenger-car equivalents per hour(PCPH)on the basis of the vehicle classification mix (i.e.,the percentage of motorcycles, autos, small trucks,buses and large combination trucks). The PCPH can be an important factor:for example,one tractor-trailer can be equivalent to up to six passenger cars