In the planning and design process, 24-hr volumes in the design year are converted to design hour volumes (dhvs) by using k factors ratio of 30th-highest-hour two-way volume to two-way average annual daily traffic (adt) , d factors (directional split during the 30th-highest hour), and t factors (percentage heavy vehicles in the dhv). Lack of precision in selection of these parameters can make a plus or minus 50% difference in the design of urban freeways. The procedures used to select these factors in texas urban areas have been examined, and refinements to the process are proposed. Using data from automatic traffic recorder (atr) stations for 1973 to 1988 in the texas cities of houston, dallas, fort worth, austin, and san antonio, along with manual classification data for 1984 to 1988, k, d, and t factors have been studied under increasingly urban conditions. Emphasis has been placed on the k and d factor research. Variables identified and tested for significance in predicting dhvs included facility type (radial or circumferential freeway), weekday adt per lane, degree of capacity utilization during the peak period, employment density near the atr station, length of peak period, and distance from thecentral business district. Although the initial data sets have yielded an unstable model over time using a multivariable regression analysis, ranges of k sub d (k factor by direction) for ranges of each variable are developed for use in determining reasonableness of preselected k and d values. This paper appears in transportation research record no. 1320, Freeway operations, highway capacity, and trafficflow 1991.
Abstract