In order to determine how many buses an exclusive bus lane on a freeway can carry, and the effects of buses on mixed freeway traffic flows, a study was made on the effect of buses on freeway capacity. The study was done by cluster analysis. A cluster is a group of three or more vehicles of the same type with a time headway between individual vehicles of 0.0020 hr or less. Headway was measured from the front axle of a vehicle to the front axle of the following vehicle. Average cluster values were found within each of several different speed ranges. It was assumed that the average cluster values would represent values approaching possible capacity, while practical capacities would be found at the median point of the cluster values. Capacity values obtained by cluster analysis showed good agreement with those by other methods, and cluster analysis was shown useful for determining lane capacity. Possible capacity, as defined by the highway capacity manual, represents the highest volume reached on a given facility, regardless of the speed at which this occurs. Separate capacity values are reported for each of the several speed ranges analyzed, each refers to the maximum volume that can be expected within that speed range. The highest volumes that can be expected to be found within each of the different speed ranges were determined by the cluster analysis. All values were adjusted to a 12-ft lane with no lateral restrictions. The bus equivalency factor for a bus on a reasonably level freeway may be taken as 1.6 regardless of which lane it is in or the speed at which it is traveling. The possible capacity of an exclusive bus lane is 1,300 buses per hour at 25 and 30 mph on such a freeway. Graphs of bus volume rates versus auto volume rates under various conditions are given. The use of exclusive bus lanes of freeways is discussed.
Abstract