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Table A-III: Urban Street Class based on Function and Design Categories Desi n Cate o Functional Category g g ry PrlnclpalArterlal Minor Arterial High-Speed I Not applicable Suburban II II Intermediate II III or IV Urban III or IV IV Source: Highway Capacity Manual 2000 Table A-IV: Urban Street Levels of Service by Class Urban Street Class 1 11 111 IV Range of Free Flow Speeds (mph) 45 to 55 35 to 45 30 to 35 25 to 35 Typical Free Flow Speed (mph) 50 40 33 30 LOS Average Travel Speed (mph) A >42 >35 >30 >25 B >34 >28 >24 >19 C >27 >22 >18 >13 D >21 >17 >14 >9 E >Ib >13 >10 >7 F 516 <13 <_10 57 Source: Highway Capacity Manua12000 Interrupted Flow One of the more important elements limiting, and often interrupting the flow of traffic on a highway is the intersection. Flow on an interrupted. facility is usually dominated by points of fixed operation such as traffic signals, stop and yield signs. These all operate quite differently and have differing impacts on overall Flow. Signalized Intersections The capacity of a highway is related primarily to the geometric characteristics of the facility, as well as to the composition of the traffic stream on the facility. Geometrics are a fixed, ornon-varying, characteristic of a facility. At the signalized intersection, an additional element is introduced into the concept of capacity: time allocation. A traffic signal essentially allocates time among conflicting traffic movements seeking use of the same physical space. The way in which time is allocated has a significant impact on the operation of the intersection and on the capacity of the intersection and its approaches. LOS for signalized intersections is defined in terms of control delay, which is a measure of driver discomfort, frustration, fuel consumption, and increased travel time. The delay experienced by a motorist is made up of a number of (actors that relate to control, traffic and incidents. Total delay is the difference between the travel time actually experienced and the reference travel time that would result during base conditions, i. e., in the absence of traffic control, geometric delay, any incidents, and any other vehicles. Specifically, LOS criteria for traffic signals are stated in terms of average control delay per vehicle, typically fora I S-minute analysis period. Delay is a complex measure and depends on a number of variables, including the quality of progression, the cycle length, the ratio of green time to cycle length and the volume to capacity ratio for the lane group.