Joy Dahlgren

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.

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.

Enhancement of Vehicle Speed Estimation with Single Loop Detectors

Traffic data from single loop detectors are a dominant data source widely used in traffic operation centers and traveler information systems. Speed estimation from single loop detectors is based mainly on occupancy data, a conversion factor from occupancy to density (which is related to vehicle length), and the assumed relationship between flow, speed, and density. The discrepancy between the speed estimated with single loops and the speed measured directly from double loops was investigated. It was found that the inaccuracy of speed estimation done with single loops was caused mainly by the irregular behavior of vehicle pace. Under congested or unstable traffic conditions, the distribution of vehicle pace within a given time interval often exhibits a large variance accompanied by a strong skewness. Accuracy in speed estimation can be improved by computing occupancy in a different way—by using median vehicle passage time over the detector—instead of mean vehicle passage time, often used in the conventional method. The performance of the enhanced speed estimation method is very encouraging. The use of the median vehicle passage time reduces the skewness of pace data.

A planning methodology for intelligent urban transportation systems

Urban transportation Planning is facing challenges and opportunities in the rapid developments of intelligent transportation systems. Such systems are characterized by real time information feedback in their operations and management, and by increasing levels of automation of their various components. The challenges to planning stem from the increased range and added complexity of the choices available to transportation planners. The implementation of IVHS technologies, many of which have system-wide implications will require a change in the institutional arrangements that are currently at work m transportation planning. Recent legislation, such as ISTEA and the California Congestion Management Program, has also posed a challenge to transportation planning as it requires specific processes and imposes certain mandates.

Benefit-Cost Evaluation of the Electronic Toll Collection System: A Comprehensive Framework and Application

Although electronic toll collection (ETC) technologies are fairly mature and many ETC systems have been implemented, there is a dearth of comprehensive and systematic assessment of their benefits and costs. Most previous studies have focused only on one or two short-term benefits or costs and have not translated the benefits into dollar values. A comprehensive framework is proposed, incorporating various benefit and cost components, and decomposing the benefits and costs by affected groups. The framework also is tested using data for the ETC system at the Carquinez Bridge in the San Francisco Bay Area. The results of the analyssis show that while the Carquinez ETC project would generate a benefit-cost ratio of 40 over the entire evaluation period, the distribution of costs and benefits would be uneven. Time saving is a major direct benefit of ETC. Other benefits include energy saving, emission reduction, and service improvement. Toll patrons would be the primary beneficiaries. However, cost savings to the agency would depend largely on the demand for use of the ETC system and the cost of transponders. Implications of the findings are discussed.

Dynamic Procedure for Short-Term Prediction of Traffic Conditions

Many existing models for forecasting traffic conditions are based on traffic flows. Field data are used here to show that these traffic conditions may not fluctuate from day to day in the same manner as does the traffic flow. Consequently, flow data are inappropriate for predicting traffic conditions because the same flow level may correspond to either a congested or a free-flow traffic state, a phenomenon that can be easily explained with the flow–density relationship. Occupancy, which is proportional to density, is a better indicator of traffic condition. A simple dynamic model based on occupancy data is proposed. The model utilizes occupancy and occupancy increments in an integrated way and treats them as two random variables represented by two normal distribution functions. It is shown that flow data, which are more stable than occupancy data, can be used indirectly to improve the performance of the proposed model. Self- and cross-validation efforts are made to examine the performance of the model. The results are promising. The expected absolute deviance for predicted occupancy (ranging from 0 to 100%) is about 1.25%, which is accurate enough for most applications. The model requires little effort in calibration and computation and is exceedingly simple to implement in the field.

An enhancement to speed estimation using single loop detectors

Speed estimation plays an important role in many components in intelligent transportation systems (ITS). In practice, traffic data from single loop detectors are one of the dominant data sources. Speed estimation from single loop detectors is mainly based on occupancy data, a conversion factor from occupancy to density (which is potentially related to the vehicle length), and the assumed relationship between flow, speed, and density. This paper investigates the discrepancy between the speed estimated with single loops and the speed estimated with double loops. It was found that the discrepancy between the two is mainly caused by the high variance in vehicle pace, which, especially under congested traffic, is often accompanied with the high skewness. Accuracy can be improved by computing occupancy in a different way, using the median vehicle passage time over the detector as opposed to the mean vehicle passage time often used in the conventional method. The performance of the enhanced speed estimation method is very promising. The use of the median vehicle passage time reduces the skewness of pace data.