The economic efficiency of lane differentiation policies
In congested cities throughout the world, governments are looking for ways to make better use of existing infrastructure and to limit the impacts of congestion on freight and public transport. The most common example is high-occupancy vehicle (HOV) lanes whereby a lane is reserved for vehicles with either two or more, or three or more occupants. Lane differentiation can be achieved either by regulation, decreeing which classes of road users have the right to travel in which lanes, or through pricing, charging different tolls for different lanes. The HOT-lane concept employs both methods. The present paper is concerned almost entirely with the objective of economic efficiency. The value to society of the impacts of lane differentiation is measured by summing the economic benefits and costs regardless of to whom they accrue. The paper also examines the distributional implications for different policies for traffic with different values of time. (a) For the covering entry of this conference, please see ITRD abstract no. E213716.
High-occupancy/toll lanes: where should they be implemented?
Since the first high-occupancy toll (HOT) lane was implemented in Orange County, California in 1995 there has been an increasing interest in such lanes as a means to reduce congestion, increase utilization of underutilized high-occupancy vehicle (HOV) lanes, and generate revenue. In order to determine where such lanes are appropriate, a model is constructed to estimate how adding an HOV lane, an HOT lane or a mixed flow lane to an existing freeway affects delay in various circumstances. Except when the initial delay is very high, adding a mixed flow lane is more effective in reducing delay than adding either an HOV or HOT lane, and it costs less. If both the initial delay and the initial proportion of HOVs are very high, then adding an HOV lane is more effective in reducing delay than adding either a mixed flow or HOT lane. But if the initial delay is high and the initial proportion of HOVs is not high an HOT lane is the most effective in reducing delay. Adding an HOT lane should also be considered if substantial growth in traffic is expected, because HOT lanes perform relatively well in terms of reducing delay across all levels of initial delay and HOV usage. However, because HOT lanes are considerably more costly to construct and operate than mixed flow or HOV lanes, the revenue generating potential must also be assessed, and the savings in delay must be sufficient to justify the additional cost. The revenue generating capacity of HOT lanes is low when the initial delay is low, and in such cases alternate methods of utilizing the HOT lane initially might be desirable.
High Occupancy Vehicle Lanes: Not Always More Effective Than General Purpose Lanes
The success of a high occupancy vehicle lane in motivating people to shift to carpools and buses depends on maintaining a travel time differential between it and the adjacent general purpose lanes. This differential, in turn, depends on the level of continuing delay on the general purpose lanes. Therefore, it is clear that a high occupancy vehicle lane that will motivate people to shift to high occupancy vehicles will not eliminate congestion. Consequently, it is not clear that constructing a high occupancy vehicle lane will necessarily reduce delay more than construction of a general purpose lane. The objective of this research is to determine the circumstances in which this would be the case. The hypothesis is that such circumstances would be quite limited, and this proves to be the case. The intended benefits of high occupancy lanes are defined as reduced person-delay and reduced emissions. A model is developed to calculate these benefits for four alternatives add a high occupancy vehicle lane, add a general purpose lane, convert an existing lane to a high occupancy vehicle lane, and do nothing. The model takes into account the initial conditions, the dynamic nature of the travel time differential between the high occupancy vehicle lane and other lanes, and the uncertainty regarding the extent to which people will shift modes. It combines queueing theory and mode choice theory and provides a robust method for comparing alternatives using a small amount of easily observed data. Application of the model in typical situations shows that with initial delays on the order of 15 min or more, adding a high occupancy vehicle lane would provide substantial reductions in delay and some reduction in emissions. 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 initial proportion of high occupancy vehicles falls in a rather narrow range, would an added high occupancy vehicle lane be more effective. The proportion of high occupancy vehicles must be such that it allows good utilization of the high occupancy vehicle lane while maintaining a sufficient travel time differential to motivate a shift to buses or carpools. Adding a high occupancy vehicle lane to a three lane freeway will be more effective than adding a general purpose lane only if the initial maximum delay is on the order of 35 min or more and the proportion of high occupancy vehicles is on the order of 20%. Federal policies encourage construction of high occupancy vehicle lanes and restrict funding for general purpose lanes in areas that have not attained air quality standards. The findings of this research suggest a need to reconsider these policies.