Capacity of shared-short lanes at unsignalized intersections

The calculation procedures in recent highway capacity manuals do not treat shared/short lanes at unsignalized intersections in an exact manner. The capacity of individual streams (left turn, through and right turn) are calculated separately. If the streams share a common traffic lane, the capacity of the shared lane is then calculated according to the shared lane procedure from Harders (i.e. the lengths of the short lanes are considered either as infinite or as zero. The exact lengths of the separate short lanes are not taken into account. Therefore, the capacity computed from conventional methods is overestimated, whereas that from the shared lanes' formula is underestimated. This paper presents an analytical procedure, based on probability theory, for estimating the capacity of shared and short lanes. This procedure combines the existing procedures for estimating the capacity of shared and short lanes. Tested by simulations in the style of the KNOSIMO simulation model, it can be used for arbitrary lane configurations. For simple shared/short lane configurations, explicit equations are derived for estimating the capacity. For complicated shared/short lane configurations, iteration procedures are given. As a special case, the so-called flared minor approaches are treated according to the theory derived.