High-occupancy Vehicle Research Articles

Simulation-Based Investigation on High-Occupancy Toll Lane Operations for Washington State Route 167

High-occupancy toll (HOT) lane operation has been implemented in several urban areas in the United States and is regarded as one of the most effective management strategies against freeway congestion. By allowing single occupancy vehicles (SOVs) to pay a toll for using high-occupancy vehicle (HOV) lanes, the excess capacities of HOV lanes can be used and the overall traffic mobility of the roadway section can be improved. However, research on HOT lane operations is still in its early stage. A series of theoretical and practical issues on optimizing HOT lane system performance particularly require immediate attention for better practice. Simulation-based investigation on HOT lane operations provides a cost-effective, risk-free, and prospective means of exploring and addressing these issues. A microscopic traffic simulation tool, VISSIM, is exploited in this study. By overcoming functional constraints with the VISSIM built-in modules and taking advantage of its component object model interface, we develop a new external module to enable HOT lane simulation. This HOT lane module provides additional flexibility to satisfy any specific demands from particular researchers and practitioners. Based on this external module, HOT lane operations can be simulated and evaluated as demonstrated using the Washington State Route (SR) 167 HOT Lane Pilot Project. The SR-167 simulation results not only quantitatively evaluate the overall system performance but also identify potential problems. Significantly different operational performance was illustrated between HOV lane and HOT lane systems using the developed simulation models. The simulation platform used in this study has the potential to be a cost-effective evaluation tool for HOT lane operations.

Improvements to On-Road Emission Modeling of Freeways with High-Occupancy Vehicle Facilities

Because high-occupancy vehicle (HOV) facilities have become an integral part of the freeway systems in many parts of the United States, transportation practitioners must estimate emissions from these facilities accurately. This study examined the differences between HOV and mixed-flow (MF) lanes relative to their traffic dynamics as described by speed and acceleration profiles of the collected driving trajectory data, as well as to their vehicle fleet composition. It was found that the model year distributions of vehicles in the two lane types at selected locations were not significantly different. However, the differences in speed, acceleration, and road load power of their driving data samples were statistically significant at each of the six levels of congestion tested (i.e., Levels of Service A through F). Hence, lane correction factors were developed for adjusting freeway emission factors to result in emission factors specific for an HOV lane. These correction factors have higher impacts on HOV emission rates of carbon monoxide (CO) and carbon dioxide (CO2) than of hydrocarbon (HC) and oxides of nitrogen (NOx). For CO and CO2, the impact can be as much as 30% and 10%, respectively, at very low speed. The difference between NOx emission rates of HOV and MF lanes is less than 5%. HC emission rates for HOV lanes are different from those for MF lanes by merely ±2%. These lane correction factors can be used to improve the accuracy of emission analyses for freeways with HOV facilities.

Investigating the Impact of Tolls on High-Occupancy-Vehicle Lanes Using Managed Lanes

For nearly 40 years, high-occupancy-vehicle (HOV) lanes have been used to combat congestion. These lanes allow only vehicles with multiple occupants and generally offer a free-flow trip, unlike adjacent general purpose lanes. In theory, this encourages additional carpooling, reduces overall vehicle miles of travel, and improves the commute trip. However, as a result of the underutilization of some of these lanes, some HOV lanes are migrating to high-occupancy-toll (HOT) lanes, where HOVs may travel free of charge but lower-occupancy vehicles can pay a toll to use the lane. This research investigated the impacts of offering preferential treatment for HOVs on these lanes. To determine the impact of different HOT lane operating strategies on their travel behavior, freeway travelers in the Houston and Dallas metropolitan areas of Texas were surveyed. A nested logit model was developed to estimate the mode choice for travelers. This model was used to predict the impact of converting an HOV lane to a HOT lane on which all travelers pay a toll. It was found that the overall percentage of HOV2 and HOV3+ vehicles in the traffic stream decreased by only a small amount when a toll was required for them to use the HOV lane. However, that decrease did represent a significant portion of those modes (more than 9%) and resulted in more than a 10% increase in HOT lane revenue. Therefore, elimination of preferential treatment for these vehicle types has significant implications and becomes a difficult policy decision—not just a straightforward choice.

Safety Performance of High-Occupancy-Vehicle Facilities

Collision data from high-occupancy-vehicle (HOV) facilities with two different types of access, continuous and limited, are examined in this paper. The findings show that HOV facilities with limited access offer no safety advantages over those with continuous access. Compared with continuous access HOV lanes, a higher percentage of collisions were concentrated on limited-access HOV lanes. Limited-access HOV lanes also had higher collision rates. Findings from investigating the relationship between collision rates in HOV lanes with respect to shoulder width, length of access, and proximity of access to neighboring ramps are also documented. These findings provide enhanced understanding about the effects of geometric factors on the collision rates in HOV lanes.

Effect of High-Occupancy Toll Lanes on Mass Vehicle Emissions

High-occupancy toll (HOT) lanes were recently proposed for I-85 in Atlanta, Georgia, as a way to relieve congestion and to provide a reliable commute time for single-occupant drivers who are willing to pay a toll. It is important to evaluate the air quality impacts of such a proposal in the context of environmental regulations, such as the National Environmental Policy Act (NEPA), and transportation conformity regulations. Several factors affect mass vehicle emissions, such as vehicle activity, speed, age distributions, and class distributions. These factors are incorporated into a base scenario that models the current conditions on I-85 with high-occupancy vehicle lanes by using data collected in the corridor during the summer of 2007, and a future scenario that models the implementation of HOT lanes on this corridor by using information from cities that already have HOT lanes. The MOBILE-Matrix modeling tool, recently developed by the Georgia Institute of Technology, is used for the emissions analysis by means of the input factors described above. It calculates mass emissions for five pollutants: hydrocarbons (HC), oxides of nitrogen (NOx), carbon monoxide (CO), particulate matter with an aerodynamic diameter of 2.5 μm or smaller (PM2.5), and PM10 (with an aerodynamic diameter of 10 μm or smaller) as a function of fleet composition and on-road operating conditions. The modeling work predicts extremely small increases in mass emissions for NOx, CO, PM2.5, and PM10, and an extremely small decrease in mass emissions for HC. In addition, the postimplementation emissions changes fall well within the motor vehicle emissions budgets for the facility that are used in air quality planning. Therefore, implementation of HOT lanes on I-85 in Atlanta should not violate the emissions budget requirements of the federal Transportation Conformity Rule. For NEPA purposes, this analysis could be used to make the case that air quality impacts are not significant, and therefore further detailed analyses are not required.

Why are Californians interested in hybrid cars?

This paper explores quantitatively Californians' interest in hybrid electric vehicles (HEV) based on a statewide phone survey conducted in July 2004 by the Public Policy Institute of California. The paper develops ordered choice models and factors that summarise beliefs about energy and the environment using principal component analysis. As expected, Californians concerned about the environment, energy efficiency, global warming and recent increases in the price of gasoline state a higher interest in hybrids; an even more important reason for considering hybrid electric vehicles, however, is the possibility of using high occupancy vehicle (HOV) lanes while driving alone, especially for people with potentially long commutes to work. The findings also suggest that beliefs about energy and the environment should be included in vehicle type choice models.

Safety Analysis of Florida Urban Limited Access Highways with Special Focus on the Influence of Truck Lane Restriction Policy

Detailed modeling of traffic crashes occurring on urban limited access highways in Florida was conducted to determine the influence of various geometric and traffic variables on crash occurrence with special emphasis on presence of truck lane restriction. The geometric variables modeled were length of each segment, number of lanes, number of interchanges, number of ramps, lane width, and shoulder width. The traffic variables modeled were average annual daily traffic, percent of trucks, operating speeds, presence of high occupancy vehicle lane, and presence of truck lane restriction lanes. Also included in the modeling were variables accounting for differences in driving behavior in different urban regions of the State of Florida. The results showed that, although statistically insignificant (p<=0.808), the presence of truck lane restriction had a negative modeling coefficient suggesting that sections with truck lane restriction tended to have less crashes than sections without truck lane restriction. This tendency was confirmed with a marginal effect analysis which showed that implementing truck lane restriction on a Florida urban freeway would have an annual effect of reducing crashes by approximately 4%. These results are in line with the results reported in a number of previous studies investigating the efficacy of truck lane restriction. In addition, various modeling paradigms revealed that there were regional differences in crash occurrence in which the probability of crash occurrence increased from the Orlando area, the Jacksonville area, the Tampa area, the to tricounty area of Palm Beach, Broward, and Miami-Dade, in that order.

An optical method for automated roadside detection and counting of vehicle occupants

The monitoring and control of traffic volume is becoming a constant social, economic, and environmental pressure in the UK and elsewhere, because of landmass and current infrastructure strain under a swelling and increasingly mobile population. The viability of high-occupancy vehicle (HOV) lanes for easing traffic congestion, and hence maximising traffic flow, has been proven in countries worldwide. The USA, Australia, and Canada have had HOV installations for some time, controlling the flux of traffic into their most densely populated areas. Experience in these cases has dictated that it is enforcement which is crucial to the successful implementation of such a traffic policy. To date, all enforcement has been manual, i.e. a police officer counting the occupants in a vehicle as it passes by. Studies have concluded that manual enforcement is typically only 65 per cent accurate and, considering the pressures which one individual is put under in these circumstances, this statistic is not surprising. Lighting and environmental conditions, skin tone, and location of the occupants and the alertness of the officer are all variables affecting the accuracy of manually collected data. Hence there is the need for an autonomous detection system to count the occupancy within a vehicle. This could provide the basis of an enforcement or congestion strategy.

Effects of high occupancy vehicle lanes on freeway congestion

Previous research on the effect of HOV (high occupancy vehicle) lanes on bottleneck flows is extended here to entire freeways using both theory and empirical evidence. The paper shows that if the flows of both high- and low-occupancy vehicles remain invariant before and after a freeway lane is converted to HOV use, then the freeway’s overall traffic density upstream of its bottlenecks is reduced – albeit less than expected – if the HOV lane is underutilized. As a result, HOV lanes can extend queues over longer distances. These expansions can be problematic if the queues’ expanded portions impede traffic on heavily traveled routes that do not pass through the bottleneck. To quantify this effect, the paper analyzes HOV lanes on long, multi-ramp freeways. Formulae are given for the changes in people-hours and vehicle-hours of travel induced by an HOV lane, both when there is uncongested freeway space upstream of the queue to accommodate its expansion, and when there is not. All the inputs to these formulae are either observable or easy to estimate. Hence, the recipes can help evaluate any freeway’s existing, or planned, HOV lane installation. The HOV lanes at all the sites we have analyzed, which are quite typical, add less than 2% to vehicular delay and reduce people delay by more than 10%. These estimates assume no increase in car-pooling. More generally, the paper also suggests how to deploy HOV lanes on city-wide freeway systems and recommends steps to better plan city-wide systems of bus lanes.

Impacts of freeway high-occupancy vehicle lane configuration on vehicle emissions

In California, high-occupancy vehicle (HOV) lanes on freeways have been implemented with two different configurations – continuous access HOV lanes, primarily used in Northern California, and limited access HOV lanes, primarily used in Southern California. Using a new emissions modeling methodology that integrates a microscopic traffic simulation model (PARAMICS) with a modal emissions model (CMEM), this study estimates and compares vehicle emissions contributed from these two types of HOV lane configurations. Under various scenarios with different levels of vehicle demand and percentage of HOVs in the traffic mix, it is found that the freeway with continuous access HOV lane consistently produce lower levels of pollutant emissions compared to the freeway with limited access HOV lane. This is primarily due to the highly concentrated weaving maneuvers that take place on the dedicated ingress/egress sections on the freeway with limited access HOV lane, which cause relatively higher frequency and magnitude of acceleration/deceleration events, resulting in higher emissions on these sections of the freeway.

High-Occupancy Toll Lanes and Public Transportation

This paper examines the impacts on public transportation services and ridership levels that have resulted from the expansion of high-occupancy vehicle (HOV) lanes to high-occupancy toll (HOT) lanes. The experience with bus services operating on the HOV and HOT lanes on I-15 in San Diego, California; I-394 in Minneapolis, Minnesota; and I-25 in Denver, Colorado is explored. The Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) allows increased flexibility for state departments of transportation and other agencies to maximize the use of HOV lanes, including use by tolled single-occupant or lower-occupancy vehicles. As a result, the potential impacts on bus services and ridership levels represent important factors when expanding HOV lane user groups to include tolled vehicles. The three projects provide different perspectives on the potential impacts on public transportation from expanding HOV lanes to include HOT vehicles, as well as the use of transit buses to monitor travel time savings and trip time reliability.

Analysis of Reduction in Effective Capacities of High-Occupancy Vehicle Lanes Related to Traffic Behavior

Previous research efforts have quantified the capacity of non-barrier-separated, high-occupancy vehicle (HOV) lanes. However, the majority of these efforts have focused on HOV lanes that usually operate well below capacity. In contrast, the I-85 HOV lanes in Atlanta, Georgia, usually operate under constrained flow conditions during morning and afternoon peak periods. This provides an opportunity to assess the relative performance of the HOV lane to the adjacent general-purpose (GP) lanes, which also undergo constrained flow conditions. This paper considers the relationship between the performance of the HOV and GP lanes by examining speed differential effects as a function of congestion on GP lanes. The research provides evidence of a sympathetic reduction in vehicle speeds in the HOV lane. This is likely explained by the safety concerns of the HOV drivers associated with the potential for vehicles using the congested GP lanes to enter the HOV lane, as well as the safety concerns of HOV drivers looking for an acceptable gap to merge into the GP lanes to access a downstream exit ramp.

Intermediate Access to Buffer-Separated Managed Lanes

A Texas Department of Transportation research project developed guidance on intermediate access to and from buffer-separated managed lanes located within general purpose lanes. For intermediate access openings, other communities recommend opening lengths between 1,300 ft and 2,000 ft (396 m to 610 m). While the recommended buffer width is 4 ft (1.2 m) or greater, many sites have a 2- to 3-ft (0.6- to 0.9-m) width. To assist in developing the guidance material, researchers recorded operations at five intermediate access sites. Characteristics measured included where the vehicle entered or left the managed lane (early, within one of two or three equal-distance zones, or late) and the lane of origin for the vehicle. A surprisingly large number of maneuvers (7%) was for vehicles passing a slower-moving vehicle. Approximately 9% of those moving into the high-occupancy vehicle (HOV) lane and 8% of those moving out of the HOV lane crossed the solid white markings (i.e., not in compliance with the pavement markings). The percentage of noncompliance increased to about 15% during those periods with low speeds [less than 40 mph (64 km/h)] or high speeds [greater than 60 mph (96 km/h)]. The percentage of maneuvers in compliance with the pavement markings varied by the length of the intermediate access opening. The compliance rate was greater for the longer access opening length [1,500 ft (457 m)] than for the shorter access opening length [1,160 ft (354 m)].

Potential Mode Shift from Transit to Single-Occupancy Vehicles on a High-Occupancy Toll Lane

Modifying a high-occupancy vehicle (HOV) lane to a high-occupancy toll (HOT) lane generally involves allowing single-occupancy vehicles (SOVs) to travel on the free-flow HOV lane for a toll. This may entice some transit riders to pay the toll to obtain the benefits of traveling in their own vehicle on the HOV lane. Thus, the introduction of a HOT lane has the potential to affect transit ridership negatively; this situation would lower the average vehicle occupancy (AVO) of the lane. To investigate this potential problem, surveys were distributed to park-and-ride bus passengers on two Houston, Texas, freeway corridors. Passengers’ responses to questions regarding their trip characteristics, their socioeconomic characteristics, and their stated preference scenarios were used to develop a mode choice model. Scenarios with varying tolls and travel time savings were simulated using this model. For all scenarios, only a small percentage of transit passengers would choose to switch to driving alone on the HOT lane. Transit passengers shifting to SOVs on the HOT lane would reduce average vehicle occupancy on the lane by only about 1% to 2%. SOV drivers shifting from the general purpose lanes to the HOT lanes are likely to affect AVO much more. However, as long as free-flow conditions are maintained, this analysis shows that the HOV lane can be successfully adapted to a HOT lane and move more people, even if a few transit passengers choose to drive alone.

Toward the Evaluation of Value Pricing

Value pricing—a form of multiclass service in which the user has the choice of an unpriced standard service or a faster service that is priced—has often been evaluated by comparison with high-occupancy vehicle lanes and also by inclusion of additional capacity as part of the project alternative. From the standpoint of benefit–cost analysis, this is not the right comparison. The question of whether to invest in additional capacity should be kept separate from the best way to operate it. Value pricing should be judged against other ways of using the same physical capacity, specifically, a single-class service either priced or unpriced. In addition, two factors not explicitly included in previous evaluations—partitioning a single facility into separate services diminishes total capacity, and capacity may be further reduced if the traffic flow goes from saturated to oversaturated (hypercongestion)—give multiclass service a significant disadvantage to overcome relative to single-class service. The net benefits of value pricing thus depend on whether the heterogeneity of vehicle time values (through occupancy or individual time values) is great enough to overcome the disadvantages of creating separate facilities for each service. The points are illustrated via a simplified numerical example that concentrates on only operational considerations and assumes a given total hourly traffic flow, so the results are no more than suggestive.

Minnesota's Proposal for Urban Partnership Agreement

In 2007 Minnesota was one of five finalists for an Urban Partnership Agreement (UPA) grant through a unique competition initiated by the U.S. Department of Transportation (USDOT). Minnesota was awarded a $133-million UPA grant by the USDOT to use congestion pricing in combination with other strategies to reduce congestion in a major transportation corridor. In 2005 Minnesota had previously adapted a high-occupancy vehicle lane to a high-occupancy toll (HOT) lane on I-394 west of downtown Minneapolis. These HOT lanes are known as MnPASS lanes. The UPA project will extend the MnPASS lanes to I-35W south of Minneapolis, the most heavily traveled stretch of urban Interstate in the Minneapolis–St. Paul metropolitan area. The proposal includes the use of priced dynamic shoulder lanes to make more efficient use of existing capacity as well as improved transit, telecommuting, and technology support in the corridor and in downtown Minneapolis. The significant federal dollar commitment to funding UPA projects, particularly through transit funding, has played a key role in generating political and institutional dialogue and support, especially among local government elected officials and legislators in the I-35W corridor. Stakeholder workshops have been an important outreach and education mechanism to inform and engage elected officials and other key constituents during the UPA application process.

Automated Vehicle Occupancy Verification Systems

Managed lanes require effective vehicle occupancy enforcement policies and programs to operate successfully. Several semi- and fully automated techniques for determining the number of persons in a moving vehicle, including operator-monitored video cameras and infrared composite imaging, have undergone limited field testing. The technical merits of invehicle and roadside systems for automated vehicle occupancy verification systems have been examined extensively. This paper complements those efforts by providing the policy and legal implications of automated systems for occupancy enforcement. The primary form of automated enforcement technology currently used in the United States is photographic imagery recorded by automated violation detection systems. For moving violations, these systems almost exclusively take the form of red light and speed enforcement, unlike toll evasion, which is typically punishable as an infraction. Principal objections to automated enforcement for occupancy purposes have involved privacy and due process concerns. Under an automated occupancy enforcement tool, the occupancy enforcement regime would likely face the same legal and privacy challenges as automated enforcement cameras and similar devices. This paper examines the current legislation related to automated enforcement practices, as well as an illustration of the arguments against their use. These arguments raise questions about the legality of automated enforcement systems and the perceived invasion of privacy some drivers may associate with such systems. The three primary privacy issues associated with an automated system for occupancy enforcement concern the photographic record of occupants, vehicle infrastructure integration barriers, and the legal definition of a high-occupancy vehicle infraction.

Effectiveness of California’s High Occupancy Vehicle (HOV) system

The effectiveness of California’s 1171 mile High Occupancy Vehicle (HOV) system is assessed using peak hour traffic data from 700+ loop detector stations over many months. The study reaches the following conclusions. (1) HOV lanes are under-utilized: 81% of HOV detectors measure flows below 1400 vehicles per hour per lane (vphpl) during the PM peak hour. (2) Many HOV lanes experience degraded operations: 18% of all HOV miles during the AM peak hour and 32% during the PM peak hour have speeds below 45 mph for more than 10% of weekdays. (3) HOV lanes suffer a 20% capacity penalty, achieving a maximum flow of 1600 vphpl at 45 mph vs. maximum flow above 2000 vphpl at 60 mph in general purpose (GP) lanes. (4) HOV lanes offer small travel time savings. The mean savings over a random 10-mile route on an HOV lane vs. the adjacent GP lane is 1.7 min and the median is 0.7 min; however, HOV travel times are more reliable. (5) Travel time savings do not provide a statistically significant carpooling incentive. (6) A system with one HOV lane and three GP lanes carries the same number of persons per hour as a system with four GP lanes. (7) HOV lanes reduce overall congestion slightly only when the general purpose lanes are allowed to become congested. Despite these findings, HOV facilities can play a useful role in a well-managed freeway system in California. In particular, they can be useful if there is a significant number of buses or vanpools; as a 2-lane HOV/HOT facility, which eliminates capacity loss; and, with efficient metering, as a HOV/HOT bypass at the on-ramps.

Evaluation of Transportation Infrastructure Management Strategies Using Microscopic Traffic Simulation

This paper presents a traffic simulation approach to evaluating the relative benefits of transportation management strategies aimed at maximizing the utilization of road infrastructure. The approach is demonstrated using a case study of an infrastructure upgrade of a commuting corridor in Brisbane, Australia. A detailed traffic simulation model was developed for the network under consideration, and a number of different infrastructure management strategies were investigated. These included treatment of the additional road capacity as a bus lane (available for buses only), a high occupancy vehicle lane, and a general traffic lane open to all vehicles. The simulation results showed that a bus lane would produce the best travel time benefits for buses (19% decrease in travel times and 68% improvement in travel time reliability). This, however, was achieved at the expense of reduced benefits to other road users. The work reported in this paper demonstrates the feasibility of using traffic simulation to quantify the benefits of alternative infrastructure strategies based on modeling of individual vehicles and their interactions on the road network.

Effects of HOV lanes on freeway bottlenecks

High occupancy vehicle (HOV) lanes are restricted-use freeway lanes reserved for vehicles with more than a predetermined number of occupants. This paper examines the physics of HOV lanes placed on median lanes, with open access everywhere. HOV lanes can affect the capacity of freeway bottlenecks through both an under-utilization effect and a disruption effect. An under-utilized HOV lane passing through a bottleneck obviously discharges less flow than possible. But lane changes in and out of the HOV lane can also disrupt the flow on the adjacent general purpose (GP) lanes, and reduce their discharge rate as well. Bottleneck capacity reductions arising from the combination of both effects are undesirable because they increase vehicle-hours of travel. This paper shows with systematic simulations that the disruption effect is not noticeable at isolated bottlenecks. If anything, HOV lanes seem to have a smoothing effect that increases the GP discharge flows at these locations. Reductions in GP-capacity were found only in highly idealized situations without bottlenecks. Thus, it appears that the total discharge rate of bottlenecks with and without HOV lanes can be conservatively analyzed assuming that the capacity of the GP lanes is not affected by the HOV lane. Using this assumption, the paper then shows how to estimate total bottleneck capacity, how to diagnose problems with existing HOV lanes, and how to deploy new ones without creating new bottlenecks or changing the total flow through existing ones. This milestone is sometimes sufficient to guarantee no change in the total vehicle-hours of travel and, hence, a reduction in people-hours of delay. The paper also examines a dynamic operating strategy for HOV lanes that increases a bottleneck’s total discharge rate by exploiting the smoothing effect. This suggests that dynamic strategies can be used to reduce not just people-hours of delay but also vehicle-hours and their externalities.