It is well known among HOV practitioners that the success of a high occupancy vehicle (HOV) lane in motivating people to shift to HOVs depends on maintaining a travel time differential between it and the adjacent general purpose lanes. This differential exists only if there is continuing delay on the general purpose lanes. The paradox inherent in this requirement – that HOV lanes as a congestion reduction measure require the continuance of congestion – is rarely noted. Because of this requirement for continuing congestion, it is not clear that construction of an HOV lane will always reduce delay or vehicle emissions more than construction of a general purpose lane. The objective of this research was to determine the circumstances in which this would be the case. The hypothesis was that such circumstances would be quite limited, and this proved to be the case.
A model was developed to calculate person-delay and emissions for four alternatives: add an HOV lane, add a general purpose lane, convert an existing lane to an HOV lane, and do nothing. The model required relatively few inputs: the beginning and ending time of the congested period, the time of the maximum delay, the length of the maximum delay, the number of lanes and the capacity per lane, the proportion of HOVs, and the average occupancy of HOVs and non-HOVs (hereafter referred to as LOV3 for low occupant vehicles). Application of the model in typical situations showed that if the initial proportion of HOVs is .15 or greater, adding an HOV lane would eliminate or substantially reduce delay. However, in a wide range of such situations, adding a general purpose lane would be even more effective. Only if the initial delay is long and the proportion of HOVs falls in a rather narrow range would an added HOV lane be more effective. In these cases the proportion of HOVs must be such that it allows good utilization of the HOV lane while maintaining a sufficient travel time differential to motivate a shift to HOVs.