Such priority is most beneficial when there are large numbers of people in buses. Increased road capacity is the primary factor in reducing person- delay. When existing high-occupancy vehicles move to the HOV lane, congestion decreases on the general-purpose lanes. At some point where the proportion of high-occupancy vehicles approaches the proportion of capacity set aside for them, person-delay is minimized. For example, if about 20 percent of vehicles on a three-lane highway already have high occupancy when an HOV lane is added, then the HOV lane will be well used and still offer a slight advantage to ridesharers.
Policymakers often just presume that constructing an HOV lane is more effective in reducing pollution than constructing another general-purpose lane. However, it does not necessarily work out that way. The main source of reduced emissions is reduced congestion, not reduced trips. Multiplied by the thousands of vehicle trips that benefit from congestion reduction, these harmful emissions could be reduced by hundreds of thousands of grams per day.
In comparison, a light passenger vehicle traveling at 20 mph emits 21 grams of ROG and grams of CO on a mile trip. Eliminating one such trip certainly provides greater environmental benefit than reducing congestion for that one vehicle. But, if overall benefits of trip reduction are to exceed overall effects of reduced congestion, thousands of trips would have to be eliminated.
But the evidence shows that the addition of an HOV lane does not greatly motivate solo drivers to rideshare, so it is difficult to eliminate trips. When freeway expansion is considered, clearly the strategy that best reduces congestion will also best reduce harmful emissions. The question, then, is whether adding an HOV lane will reduce congestion more effectively than adding a general purpose lane. The advantage of adding an HOV lane is that, as long as some delay remains on the other lanes, it does reduce some vehicle trips.
The disadvantage is that the freeway is only partially used. My research shows that HOV lanes perform better than general-purpose lanes only when there is initially long delay, thus motivating solo drivers to rideshare, and the initial proportion of HOVs is fairly high, resulting in greater use of HOV lane capacity. Figures compare the effectiveness of adding an HOV lane to adding a general-purpose lane on a three-lane highway, with average peak-period person-delay as the indicator.
An average high-occupancy vehicle is assumed to have 2. The vertical axis represents the difference in average person-delay between the added HOV lane and the added general-purpose lane. When the value is positive, it means more person-delay would occur on an added HOV lane than on an added general-purpose lane so a general-purpose lane would be more effective in reducing congestion.
The right horizontal axis represents the proportion of preexisting high-occupancy vehicles on the highway before a new lane was added. The left horizontal axis is a coefficient that estimates the likelihood of solo drivers shifting to high-occupancy vehicles, depending on how much travel-time they save by using the HOV lane. The higher the number The lower the number Figures 1 and 2 show results for highways with initial maximum delays of 15 minutes and 25 minutes, respectively.
Nowhere do the graphs drop below zero on the vertical axis. That is, in all cases an added general-purpose lane would cause either the same amount of person-delay or less person-delay than an added HOV lane, even if the initial proportion of high-occupancy vehicles were high, and even if solo drivers shifted to high-occupancy vehicles easily.
A microscopic approach to modeling air quality impacts of HOV lane conversion. Shewmake S. Can carpooling clear the road and clean the air? Journal of Planning Literature ;27 4 — EPAR; Home Mission Health.
High-Occupancy Vehicle Lanes High-occupancy vehicle HOV lanes are one or more lanes of a roadway that have restrictions on use to encourage ridesharing and can reduce vehicle miles traveled VMT. According to California state law, the goals of HOV lanes are to reduce congestion and improve air quality on the State Highway System. The law states that HOV lanes are used "to stimulate and encourage the development of ways and means of relieving traffic congestion on California highways and, at the same time, to encourage individual citizens to pool their vehicular resources and thereby conserve fuel and lessen emission of air pollutants.
HOV lanes are a viable alternative, and in most cases is the only alternative, in meeting federal air quality conformity standards for capacity-increasing improvement projects in metropolitan areas. HOV facilities represent one approach being used in metropolitan areas throughout the state to respond to growing traffic congestion, declining mobility levels, air quality, and environmental concerns.
An "occupant" is defined as any person who occupies a safety restraint device, i. Signs along the highway will specify the enforcement policy for each route. These vehicles can be distinguish by a green plug-in hybrid or white clean-air decal issued by the Department of Motor Vehicles.
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