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Potential Impact of Exempt Vehicles on HOV Lanes


Defining HOV Facilities

HOV facilities represent one approach used in metropolitan areas throughout the country to help improve the people-moving capacity rather than vehicle-moving capacity of congested freeway corridors. The travel time savings and improved trip time reliability offered by HOV facilities provide incentives for individuals to change from driving alone to carpooling, vanpooling, or riding the bus.

The development and operation of HOV facilities have evolved over the past 30 years. The opening of the bus-only lane on the Shirley Highway (I-395) in northern Virginia/Washington, D.C. in 1969 and the contraflow bus lane on the approach to New York-New Jersey's Lincoln Tunnel in 1970 represent the first freeway HOV applications in the country. Today there are some 130 HOV freeway projects in the 31 metropolitan areas in North America highlighted in Figure 1.


HOV freeway projects in 31 metropolitan areas in North America

Figure 1. Metropolitan Areas with Freeway HOV Facilities.

HOV facilities are developed and operated to provide buses, carpools, and vanpools with travel time savings and more predictable travel times to encourage individuals to choose one of these modes over driving alone. As illustrated in Figure 2, the person movement capacity of a roadway increases when more people are carried in fewer vehicles. HOV facilities are usually found in heavily congested corridors where the physical and financial feasibility of expanding the roadway is limited. Supporting services, facilities, and incentives are also used to further encourage individuals to carpool, vanpool, or ride the bus.


Number of Vehicles Needed to Carry 45 People

Figure 2. Number of Vehicles Needed to Carry 45 People.

Rather than creating disincentives to discourage drivers who travel alone, HOV lanes are developed to provide a cost-effective travel alternative that commuters will find attractive enough to change from driving alone to taking the bus, carpooling, or vanpooling. HOV projects typically focus on meeting one or more of the following three common objectives.

HOV facilities on freeways or in separate rights-of-way are typically classified into four categories. These categories include busways or exclusive HOV lanes in separate rights-of-way, exclusive HOV lanes in freeway rights-of-way, concurrent flow HOV lanes on freeways, and contraflow HOV lanes on freeways. The type of HOV facility will influence management, operation, and enforcement activities.

Many of the initial HOV lanes were bus-only applications or allowed buses and vanpools. In an effort to maximize use, carpools became the dominant use group on most projects during the 1970s and 1980s. The vehicle-occupancy requirements for carpools have evolved over time. A three-person per vehicle (3+) occupancy level was initially used on many projects, but most current facilities use a two-person per vehicle (2+) carpool designation.

The benefits provided by HOV facilities have been documented in a number of different studies. Table 1 highlights examples of vehicle and person utilization of HOV lanes throughout the country. As noted below, numerous HOV facilities offer travel time savings and trip time reliability that have influenced travelers to change from driving alone to carpooling, vanpooling, or riding the bus. The HOV lanes have resulted in increasing the average vehicle occupancy (AVO) and people-moving capacity of congested travel corridors.

Defining HOV Lane Capacity

As discussed previously, the goal of an HOV facility is to provide travel time savings and trip travel time reliability to buses, vanpools, and carpools, to encourage individuals to change from driving alone. Vehicle eligibility requirements and vehicle-occupancy requirements are typically established at levels that encourage use of the facility and the formation of new carpools, but that will not create demand high enough to make the lane congested. The challenge to operating agencies is to maintain traffic flow levels that provide the travel time savings and the trip time reliability bus riders, vanpoolers, and carpoolers come to expect.

State departments of transportation and other agencies responsible for operating HOV facilities use different measures and techniques to help monitor the operation of HOV facilities and to determine when an HOV lane is becoming too congested. There are two typical measures used; 1) vehicles per hour per lane (vphpl) and 2) average speeds.

The National Cooperative Highway Research Program HOV Systems Manual identified that volumes of 1,200 to 1,500 vphpl on most types of HOV facilities will begin to experience degradations in travel time savings and travel time reliability. The manual notes that the maximum flow or capacity will vary by facility. Some HOV lanes serving primarily carpools are operating successfully with up to 1,700 or 1,800 vphpl during the peak hour. Others, like the bus-only contraflow lane approaching the Lincoln Tunnel, reach capacity at 700 to 800 vphpl. Caltrans uses 1,650 vphpl as the maximum threshold for freeway concurrent flow facilities (6).

The manual identifies the following general maximum operating thresholds for different types of HOV facilities based on national experience.

The updated American Association of State Highway and Transportation Officials (AASHTO) Guide for High-Occupancy Vehicle Facilities (7) reflects similar maximum ranges, with two exceptions. The high end of the maximum ranges for freeway, exclusive reversible lanes and freeway concurrent flow lanes are identified as 1,600 vphpl.

A second approach to identify capacity problems is to monitor travel speeds in an HOV lane and travel-time reliability. The Washington State Department of Transportation (WSDOT) uses a guide that HOV lane vehicles should maintain or exceed an average speed of 45 mph or greater at least 90 percent of the time they use the lane during the peak hours, measured for a consecutive six-month period (8).

FHWA's Office of Operations has been developing and tracking congestion performance measures at the national level. FHWA uses congestion measures focusing on the average duration of congested travel, the travel time index, and the buffer index. There are two measures addressing the average duration of congested travel. The first is that for any five-minute interval a trip is congested if its duration exceeds 130 percent of free-flow or un-congested duration. The second measure is that if more than 20 percent of all trips in the network are congested in any five-minute time interval, the entire network is congested for that time interval.

The travel time index is defined as the ratio of congested and un-congested travel times averaged over all congested trips. The buffer index is defined as the ratio of total travel budget required for 95 percent on-time reliability over the un-congested travel time averaged over all congested trips. The buffer index provides a measure of not only how congested the system is, but also how reliable the system is. It provides a performance measure from the customer's perspective. The buffer index represents the amount of time commuters need to build into their trip to arrive at their destination on time 95 percent of the time.

It is important to note that numerous factors may influence the capacity of an HOV lane. Factors which may influence the capacity of an HOV lane include the type of HOV facility, the design, the number and the design of access points, the terminus design, traffic volumes in the general-purpose lanes, design and access elements of the general-purpose lanes, local conditions and perceptions, and the goals and objectives of a project. Information on how these factors may influence the capacity of an HOV lane is summarized next.

Options for Using Available HOV Lane Capacity

A number of options may be appropriate for consideration by operating agencies if there is available capacity in an HOV lane. These options include allowing other categories of HOVs and lowering the vehicle-occupancy requirements. Other possible alternatives include using pricing to permit lower-occupant or single-occupant vehicles to use the lanes. Still other alternatives include allowing environmentally friendly vehicles and special user group vehicles to use the HOV lanes. Law enforcement, emergency services, public service, and public transportation vehicles not meeting the occupancy requirements are examples of possible special user groups. Allowing trucks and commercial vehicles to use an HOV lane is another alternative, although design and safety issues typically limit consideration of these types of vehicles.

These potential options are briefly described in this section. More detailed information on the use of HOV lanes by environmentally friendly vehicles, and law enforcement and designated public transportation vehicles is provided in Chapters Three and Four. Table 2 highlights some of the issues and limitations that may be encountered with the use of these approaches, as well as possible advantages.

Allowing Other Categories of HOVs. A first approach to consider if there is available capacity in an HOV lane is allowing additional types of HOVs that may currently be excluded. If carpools and/or vanpools are not currently allowed to use an HOV facility that has available capacity, these two classes of HOVs would be logical to consider first. Potential issues with this approach include design or operational issues that limit use by carpools and vanpools, and the potential that demand will exceed the available capacity. Advantages of this approach include maintaining the HOV goals and objectives of a project, encouraging mode change, and supporting air quality improvement efforts.

Table 2. Potential Issues and Advantages Associated with Options for Using Available HOV Lane Capacity.
Option Potential Issues/ Limitations Potential Advantages
Allow other HOVs, such as carpools in a bus- and vanpool-only lane
  • Design limitations.
  • Operational limitations.
  • Demand may exceed capacity and overload the lane.
  • Maintains HOV goals/objectives.
  • Supports air quality efforts.
  • Encourages mode change.
Lower Occupancy Requirement
  • Demand may exceed capacity and overload the lane.
  • Operational limitations.
  • Few applications, as most HOV projects use 2+ requirement.
  • Maintains HOV goals/objectives.
  • Supports air quality efforts.
Tolled/Priced Vehicles
  • Design and cost associated with tolling infrastructure and operation.
  • Demand may exceed capacity.
  • May not support HOV goals/objectives.
  • Equity concerns.
  • Enforcement.
  • May generate significant revenues.
  • May build support for HOV lanes among additional user groups.
Environmentally Friendly Vehicles
  • Demand may exceed capacity and overload the lane.
  • Public perception.
  • Enforcement.
  • May not support HOV goals/objectives.
  • Possible equity concerns.
  • May encourage purchase of environmentally friendly vehicles.
  • May help improve air quality.
Law Enforcement Vehicles (Law enforcement, Fire, EMS)
  • Defining allowed vehicles.
  • Does not support HOV goals/objectives.
  • Public perception.
  • Enforcement.
  • May enhance response to emergencies.
Designated Public Transportation Vehicles
  • Defining allowed vehicles.
  • Public perception.
  • May be few opportunities due to transit orientation/operation.
  • Additional benefits and cost savings for transit operators.
  • Service enhancements for riders.
  • May increase ridership.
Trucks and Commercial Vehicles
  • Does not support HOV goals/objectives.
  • Design limitations.
  • Additional cost to operate.
  • Crash/safety concerns.
  • May help separate trucks from other traffic, which may provide safety benefits.

Lowering Vehicle-Occupancy Requirements. A second approach for addressing available capacity is to lower the vehicle-occupancy requirement. The application of this approach is limited, as most HOV lanes currently use a 2+ designation. The few HOV facilities that use a 3+ designation do so primarily because the facility would be too congested at the 2+ level.

The San Bernardino Freeway busway provides the best recent example of the possible consequences of lowering the vehicle-occupancy requirement from 3+ to 2+, as required by state legislation. The change, which was implemented in January 2000, resulted in the HOV lane becoming too congested. Peak-hour travel speeds on the busway were reduced from 65 mph to 20 mph, while peak hour travel times increased by 20 to 30 minutes, and bus on-time performance declined significantly. While peak-hour vehicle volumes in the HOV lane increased from 1,100 to 1,600, the number of persons carried declined from 5,900 to 5,200. There was a vocal negative response from HOV lane users, especially bus riders. At the same time, no significant improvements were realized in the general-purpose freeway lanes. Based on the negative effects on the busway, emergency legislation was approved increasing the vehicle occupancy requirement back to 3+ during the morning and afternoon peak periods effective July 24, 2000 (9).

As demonstrated by the El Monte Busway example, the main potential issue with lowering the vehicle-occupancy requirement is that an HOV lane will become too congested. In addition to degrading the travel time savings and trip time reliability HOV lane users have come to expect, this approach may cause 3+ carpools to disband and/or influence bus riders to change to 2+ carpools. The application of this option is also limited in that the majority of HOV lanes already use a 2+ requirement.

Tolled or Priced Vehicles. Another possible approach is to allow lower or single-occupancy vehicles to use an HOV facility for a fee. This technique is commonly referred to as value pricing or HOT lanes. Value pricing is currently in use on the I-15 HOV lanes in San Diego and the Katy and the Northwest HOV lanes in Houston. The I-15 project allows single-occupancy vehicles to use the HOV lanes, while the two projects in Houston allow two-person carpools to use the HOV lanes during the 3+ restricted periods for a fee. The toll lanes on SR 91 in Orange County, California provide a reduced toll charge to 3+ carpools. Other toll facilities around the county provide carpools with lower toll fees. Value pricing projects are being considered and implemented on HOV lanes in Minneapolis, Denver, Seattle, and other areas.

Potential advantages of this technique include maximizing use of available capacity, managing demand, expanding the eligible user groups, addressing real or perceived low use levels, and generating new revenues. Possible issues include enforcement, initial costs of installing and operating the toll collection facilities, adding too many vehicles to the lane, and equity issues.

Environmentally Friendly Vehicles. Another option to address available HOV capacity is allowing environmentally friendly vehicles to use the lane without meeting the occupancy requirements. As discussed in Chapter Three, federal legislation allows states to authorize ILEV use of HOV lanes without meeting minimum occupancy requirements. ILEVs were defined through EPA rulemaking in 1993 as vehicles meeting specific low-emission vehicle exhaust emission standards and also having low levels of evaporative emissions. The definition was intended to limit ILEVs to vehicles that operate on a single dedicated non-gasoline fuel, such as electricity, compressed natural gas (CNG), and liquefied natural gas (LNG). Hybrid vehicles, which operate using a combination of gasoline and electricity, do not qualify as ILEVs. At least 10 states currently have authorizing legislation related to providing HOV exemptions to ILEVs. The ILEV program is no longer an active EPA initiative.

Possible issues associated with providing HOV exemptions for environmentally friendly vehicles include demand exceeding the capacity of the lane, enforcement, public perceptions, and potential equity issues. These issues are described in more detail in Chapter Three. Potential advantages of this approach include adding new user groups, encouraging the purchase and use of these types of vehicles, and improving air quality.

Law Enforcement and Emergency Vehicles. Most state and local policies allow marked (rooftop emergency lights and sirens) law enforcement and emergency vehicles to use HOV lanes without meeting the occupancy requirements. Police, EMS, fire, and other enforcement and emergency vehicles are typically included in this category of exempt vehicles. As described in more detail in Chapter Four, there are relatively few issues when this user group is restricted to marked law enforcement and emergency vehicles.

Issues may arise, however, when the definition of allowable vehicles is too vague or the proper definition is not enforced and law enforcement and emergency personnel traveling alone in their personal vehicles or in unmarked agency vehicles when not on duty use the HOV lanes on a regular basis. This misuse may result in overloading the lane, public perception that the vehicle-occupancy requirements are not being enforced, and the need for more enforcement.

Designated Public Transportation Vehicles. Buses carrying passengers are an important part of most HOV systems. Allowing designated public transportation vehicles to use HOV lanes when they do not meet the occupancy requirement may be one approach to using available capacity. Potential HOV exemptions for these types of vehicles are described in more detail in Chapter Four.

Public transportation buses that are dead-heading or are out-of-service currently use HOV lanes in most areas. Providing access to these vehicles is not an issue in most areas since the total number of buses is relatively small and the potential to use HOV lanes in the off-peak direction of travel is limited in many cases. Potential benefits of this approach include cost savings and enhanced operating effectiveness for transit systems and improved service for riders. These benefits may result in increased transit ridership. Issues may arise however, if private transportation vehicles, such as taxicabs, airport shuttles, and similar vehicles are provided with occupancy exemptions. These issues are described in more detail in Chapter Four.

Allowing Truck and Commercial Vehicle Access. The potential use of HOV lanes by trucks during all operating hours or just the off-peak periods has been suggested in a few areas around the country. Potential issues to examine in considering truck use of an HOV facility include the type of HOV facility, access, design limitations, safety concerns, and the potential benefits to commercial vehicle operators. HOV lanes and access facilities may not be designed to accommodate commercial vehicles and there may be geometric limitations that prohibit trucks from using a facility. Safety concerns may include trucks veering across general-purpose lanes to access an HOV lane and conflicts between HOVs and trucks. Finally, truck use may increase the costs associated with operating an HOV facility if additional personnel are needed to monitor a facility or if operating hours are extended. Truck use of HOV lanes may also cause pavements to deteriorate faster.

Analyzing HOV Exemption Policies on Traffic Flow

As noted previously, the FHWA Program Guidance on HOV Operations identifies the circumstances under which federal action is required to initiate changes in the operation of an HOV facility, and the federal review process and requirements to be used in these situations. The Program Guidance identifies the information to be included as part of a federal review. Examples of needed information include original studies and plans for the HOV facility, project agreements, commitments made in the environmental process, operational assessments, analysis of future conditions, examination of alternative operating scenarios, and possible impacts on air quality levels and plans. The Program Guidance further outlines the federal review requirements related to air quality conformity, the state implementation plan, the congestion management system, the National Environmental Policy Act (NEPA) process, and other issues (1).

The Program Guidance and other available documents support the need to examine HOV systems on a regional, not just individual project, basis. Elements in this approach include a multi-year regional HOV system strategic plan, which is integrated into the metropolitan area long-range plan, and a multi-agency program to manage implementation of the system plan and to support day-to-day operation of HOV facilities and supporting services. This approach allows for the long-term regional commitment for infrastructure improvements, the careful phasing of operating segments, and coordinating the development and operation of supporting services, facilities, and policies.

The literature review and follow up e-mails and telephone calls did not identify specific tools for use in estimating the potential impacts of exempt vehicles on traffic flow in an HOV lane prior to making a decision concerning exempt vehicle use of an HOV facility. As noted previously, some state departments of transportation and other agencies use maximum operating thresholds or travel speeds and trip time reliability measures to assess current operations of HOV facilities. These measures, such as the WSDOT guide that HOV lanes should maintain or exceed an average speed of 45 mph or greater at least 90 percent of the time during the peak hours over a consecutive six-month period, can be used in assessing potential exempt vehicle policies. Soon to be available documents supported through the FHWA pooled-fund HOV study should be of benefit in conducting assessments of potential exempt vehicle policies. The documents include the HOV Eligibility Requirements and Operating Hours Handbook; the HOV Monitoring, Evaluating, and Reporting Handbook, and the HOV Enforcement Handbook.

The current use of the HOV lanes in northern Virginia by hybrid vehicles is being monitored by the Metropolitan Washington Council of Governments (WASHCOG) and the Virginia Department of Transportation (VDOT). The results of this monitoring effort, which are discussed in Chapter Three, have been used to assess the current impact of hybrid vehicle use of the HOV lanes and to estimate future impacts. Information from this monitoring effort, which started in 2003, may be of use in other areas considering exemptions for environmentally friendly vehicles. Also described in Chapter Three is a Florida Department of Transportation (FDOT) sponsored analysis of the potential impact of allowing ILEVs to use the I-95 HOV lanes in the Miami/Fort Lauderdale area. Information available from the U.S. Department of Energy on the estimated number of alternative fueled vehicles (AFVs) by state was examined with this assessment. The study estimated that AFVs accounted for approximately 0.11 percent of all vehicles in Florida in 2000. Some 64 percent of the estimated 13,330 AFVs in the state in 2000 were fueled by liquefied petroleum gas (LPG), followed by natural gas (CNG and LNG) vehicles at 24 percent (10).

The following elements may be appropriate to consider in assessing the potential influence of HOV exemptions on traffic flow on HOV facilities.

Current HOV Vehicle Volumes. The first step is obviously to examine the current vehicle volumes in the HOV lane to determine if there is available capacity for additional vehicles. Most state departments of transportation or other operating agencies monitor use of HOV lanes. Both current vehicle volumes and historical data should be examined to determine trends in use levels. If no monitoring program is in place, data on vehicle volumes, vehicle types, and vehicle-occupancy levels should be collected and analyzed.

Identify Current Numbers of Exempt Vehicles. A second step is to identify the current number or estimated number of exempt vehicles being considered. The Alternative Fuels Data Center website maintained by the U.S. Department of Energy includes a variety of information on alternative fueled vehicles and can be used in estimating existing and potential markets. The database includes information on the alternative fuels defined by the Energy Policy Act of 1992. The alternative fuels included are biodiesel, electricity, ethanol, hydrogen, natural gas, and propane. Available information of includes the estimated number and type of alternative fueled vehicles by state, the type of fuel by region, and forecasts by region (11). Appendix A presents the estimated number of alternative fueled vehicles in use by state for the three years from 2001 to 2003. These figures do not include gasoline-electric hybrid vehicles.

Additional information may be available from the state DMV or other state agencies. Dealers selling hybrid and other environmentally friendly vehicles represent another possible source of information. This information may provide a general idea of the number of environmentally friendly vehicles by county or other geographical boundary. The information may not be available at a level that will help identify the potential number of vehicles in a corridor, however. Information on law enforcement, emergency, and designated public transportation vehicles may be obtained from the appropriate local, state, and federal agencies in the area.

Estimate Growth in Number of Exempt Vehicles. A third step is to identify the anticipated growth in the exempt vehicles being considered. The Alternative Fuels Data Center Internet site includes projections by regions and projected sales by technology. Baring major breakthroughs in technology, the projections for the sale of alternative fueled vehicles – including ethanol flex, CNG bi-fuel, and LPG bi-fuel – are relatively constant. The number of hybrid models available and the sale of hybrid vehicles is projected to increase, however (11). Trend information on the purchase of these vehicles may provide an indication of future growth. The experience in Virginia highlighted in the Chapter Three also provides an indication of the potential growth in the purchase of hybrid vehicles.

Analyze Potential Impact on an HOV Lane. Adding the anticipated number of exempt vehicles in a specific HOV lane to current vehicle volumes will provide an indication of potential impacts on traffic flow in the HOV lane. The estimated growth in HOVs and exempt vehicles can be examined to gauge potential future impacts on an HOV facility.

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