FREEWAY CAPACITY DROP AND THE DEFINITION OF CAPACITY

Abstract

Two aspects of the definition of freeway capacity are considered here. The first is whether there is a reduction of maximum flow rates when a queue forms. There appears to be roughly a 6% reduction in maximum flow rates after the onset of congestion, but not of the type discussed in the current Highway Capacity Manual (HCM). An indirect issue arises during this analysis pertaining to the question of where capacity can properly be measured. The answer, not surprisingly, is that it can only be measured in a bottleneck, and not in a queue. However, the HCM discussion identifies the potential capacity drop on the basis of operations in a queue. The analysis contained here explains why that is inappropriate. The second aspect considered is the distribution over time of maximum flows at a single location. The distribution approximates a normal one well, with a mean of 6,071 and a standard deviation of 262 vehicles per hour. The unanswered question from the analysis is what portion, or percentile, of the distribution is appropriate to use to meet the HCM definition of capacity, which calls for a value that can reasonably be expected to be achieved.