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• #### References 69-70

The National Academies of Sciences, Engineering, and Medicine
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s Chapter Two PREVIOUS WORK This chapter summarizes the work of Free researchers or groups that constitute the major body of research conducted onAWSC'nt~sectionsto date. lacquesHebert published a study of Free Intersections in Chicago that became the basis for the capacity guidelines of the 1985 HCM. Anthony Richardson developed a queuing mode} usage headway data measured by Hebert. Researchers at the University of Idaho developed emp~cally-based models for capacity and delay that were published In Transportation Research Circular (TRC) 373 and In the 1994 HCM update. HEBERT AND THE 1985 HCM Jacques Hebert (1963) proposed a capacity model for AWSC intersections based on saturation headway values for two cases, one In which the driver faced another vehicle on the opposing or conflicting approaches and one In which He driver faced no over vehicles. He measured values of 4.0 seconds and 7.6 seconds for these two cases, based on a study of three sites in Chicago conducted during periods of continuous queuing. Saturation headway values for these Ho cases are He basis for He 1985 HCM capacity procedures. Hebert presented equations to forecast He capacity of an approach based on the volume split between the two intersecting sheets and He number of lanes on each street. Equation ~ gives He capacity equation four an intersection with one lane on each approach. 3600 c = - 10.15 - up (1) where vp is the proportion of He intersection volume on the higher volume street. The 1985 HCM summanzed Hebert's work into capacity guidelines that can be used to assess He performance of an AWSC Intersection based on demand split for 2-lane by 2-lane (i.e., one lane in each direction), 2-lane by 4-lane, and 4-lane by 4-lane intersections. Tables ~ and 2 show the intersection cap acid by demand split for a 2 by 2 Intersection and by intersection type for a 50/50 demand split. Table ~ shows that He intersection capac~h,r is maximum when the flow is evenly split among all approaches; He approach cap recite under this split is 425 vph. Table 2 shows that simultaneous movement from a two-lane approach can occur, increasing the capacitor of the approach. Table 1. Intersection Capacibr as a Function of Volume Split ............... :~. ~i ,.,.,.~. .,. ~, SO/SO 1,900 SS/4S 1,800 60/40 1,700 6S/3S 1,600 70/30 1,SOO Note: Demand Split is Me proportional split of Traffic between the ~n~g~ Table 2. Intersection Capacity as a Function of Intersection Type . ~........... ................. , .. ........ ,.; ....................................................................................................................... ......................................................................................................................... 2-lane by 2-lane 1,900 2-lane by 4-lane 2,800 I Plane by 4-lane 3,600 Note: Caries are based on SO/50 demand Split RICHARDSON MODEL Anthony Richardson (1987) proposed a method of computing He capacity of an approach of an AWSC intersection by iteratively computing the service time of each approach based on the likelihood of vehicles being present on the opposing and conflichug approaches. His iterative method assumed two states, no vehicles on the opposing or conflicting approaches and one or more vehicles on the conflicting approaches. The computational method converges quickly and is easily implemented using computer software. While the basic method is valid, the major 1imitadon of the Richardson mode} is He small number of sates Hat are considered. Richardson proposed an MIG/l queueing model to estimate delay, using He Pollaczek-Khintchine formula.

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