Access regulations: High occupancy vehicle/Low-emission lanes
High Occupancy Vehicle (HOV) / low-emission lanes are roadway lanes dedicated to HOVs and low-emission vehicles, along other general-use lanes. These lanes are expected to have lower congestion levels than general-use lanes, providing the opportunity to skip congestion for the eligible users. This measure is implemented to maximise person flow and manage congestion, which can lead to travel time savings and improved travel reliability. It can support carpooling practices as well as the adoption of low-emission vehicles. HOV / low-emission lanes can be implemented at various times throughout the day (e.g. during peak congestion hours) or permanently. They may allow for travel of specific vehicle technologies (e.g. hybrid, electric, low-emission or alternative fuel vehicles) and/or of vehicles showing a specific vehicle occupancy rate (e.g. 2, 3 or more persons per vehicle). Such lanes may be accessed at specific dedicated entry points or at any point where they are put in place.
This measure is a variation to congestion lanes, which give access to vehicle drivers that are willing to pay a (higher) road (or lane) user charge. In some cases, the principle of HOV / low-emission lanes and congestion lanes are combined into High-Occupancy Toll (HOT) lanes. In these cases, HOV / low-emission lanes are also available for other vehicles paying the HOT charge. Converting a HOV lane into a HOT lane can limit the positive effects of the HOV lane, but will bring revenue gain for the (toll) road operator.
It is important to note that access regulations for high occupancy vehicles are primarily aimed at maximising person throughput, managing congestion and providing an option for travel time savings. While this measure indirectly provides some CO2 benefits, the impact on CO2 reduction may be limited in case rebound effects occur (i.e. more people are encouraged to travel on the respective motorway, or in general, thanks to the travel time savings due to the HOV lane). In order to avoid such rebound effects, travel demand management measures should be put in place. These may also include mode shift measures that encourage the use of public transport, car sharing and more generally, sustainable mobility.
The CO2 emission effects of HOV / low-emission lanes depend on the set-up of the lanes and their use requirements. Providing continuous access to the HOV / low-emission lane seems to improve the CO2 reduction efficiency, while limited access reduces it. A study has evaluated a CO2 emission reduction of about 37% on respective road stretches based on observations in Aveiro, Portugal in 2012. Other studies that consider car fleet evolution effects as well as rebound effects conclude that the effect of the measure on CO2 emissions is not significant.
Several criteria can affect the efficiency of the HOV / low-emission lane implementation and potential charges that can be put in place. First, the congestion during peak hours must already be significant, with average speeds under 50 km/h on highways for instance to attract users. Second, the roadway must be used by regular drivers accessing dense activity centres to trigger a behaviour change. Third, the implementation of the measure should lead to significant travel time savings and increases in travel reliability.
Barriers to the implementation of HOV / low-emission lanes can be the geometric road design that does not enable lane construction, the enforcement required to prevent unauthorised vehicles to enter the lane, and concerns around an under-utilisation of the lane. The latter could create an empty, reduce acceptance of the measure and deteriorate congestion levels on the remaining lanes.
Aside from CO2 effects, average travel time reductions (and hence travel reliability increases) can be achieved where rebound effects can be kept in check.
HOV / low- emission lanes may increase CO2 emissions on a transport system level in case rebound effects (i.e. increased travel activity thanks to travel time savings) cannot be avoided. This will be even more the case where travel time savings thanks to such lanes lead to urban sprawl (which generates further increases to travel activity).
Further, if the measure is not well implemented, it can create bottlenecks on the road for accessing the HOV / low-emission lane. A direct consequence of this effect is a congestion increase generating higher local pollutant emissions and reducing traffic flow. Crash rates can also increase because of dangerous maneuvers to access or exit HOV / low-emission lanes.
Jakarta’s HOV lanes could only be accessed by vehicles with an occupancy rate of more than two passengers during rush-hour periods. As a result, the practice of drivers hiring “professional passengers” to qualify for the HOV lane became common practice and jeopardised the expected benefits of the measure.
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