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Guide for Highway Capacity and Operations Analysis of Active Transportation and Demand Management Strategies

Appendix G: Speed/Capacity for HOV/HOT Lane Strategies

This Appendix provides details on the free-flow speed and capacity adjustments associated with the HOV and HOT lane strategies.

Convert Mixed-Flow to HOV

This strategy converts one or more mixed-flow lanes to HOVs only for a fixed period of time and for a fixed set of freeway sections. This strategy, although not strictly an ATDM strategy, is included to overcome the inability to model existing HOV lanes in the current HCM 2010 freeway method.

The operation and performance of barrier separated (painted or physical), limited access HOV lanes cannot be evaluated with current HCM 2010 freeway methods. The HOV lane must be analyzed as completely integrated with the freeway, with HOVs allowed to enter or leave the HOV lane at any point.

The analyst must specify the number of HOVs plus violators that will use the HOV lane. This value can be approximated as the percent of eligible HOVs on the facility, perhaps discounted a bit in recognition that not all eligible HOVs will use the HOV lane.

The HOV lane or lanes are assumed to be on the left most lane or lanes of the freeway.

The default capacity of an HOV lane is 1,600 vehicles per hour (based on unpublished NCHRP 3-96 research). For two-lane HOV facilities a slightly higher capacity of 1,800 vph/lane may be used. These default values can be overridden in the software by the analyst.

Since, the HCM 2010 freeway method does not recognize individual lane capacities, it is necessary to compute an average capacity for freeway sections with HOV lanes, across all lanes. For the case when there are not enough eligible HOVs plus violators to fill up the HOV lanes, then the capacity of the HOV lane is set at the lower value, the number of eligible HOVs plus violators.

Equation to compute an average capacity for freeway sections with high-occupancy vehicle (HOV) lanes, across all lanes.

Where:

AveCap(s) = average capacity per lane for section(s) (vph/ln)
CapHOV(s) = Min (Capacity per HOV Lane, Eligible HOVs per HOV lane) (vph/ln)
HOVlanes(s) = Number of HOV lanes in section(s).
CapMFLanes = capacity per mixed-flow lane (vph/ln).
MFLanes(s) = Number of mixed-flow lanes in section(s)

The free-flow speed and speed-flow curve for HOV lanes are assumed to be the same as for mixed-flow lanes, with the only difference being the capacity of the HOV lanes.

HOV Lanes Opened to All

This strategy opens up the HOV lane(s) to all vehicles. It might be employed in the case of a special event, weather event, incident, or work zone.

Since the HCM 2010 method cannot evaluate barrier separated HOV lane operations, the HOV lane is assumed to be completely accessible to and from the adjacent mixed-flow lanes.

Under this strategy the HOV lanes become just like mixed-flow lanes. The capacity and free-flow speed of the HOV lanes under this strategy then reverts to that of the adjacent mixed-flow lanes.

Convert Lanes to HOT Lanes

This strategy converts one or more mixed-flow lanes to high-occupancy toll (HOT) lanes for a user specified fixed period of time and set of freeway sections.

It is assumed that the toll will be set as necessary to guarantee that the HOT lane(s) are fully utilized. Thus regardless of the number of eligible HOVs that can use the HOT lane for free (or a reduced rate) it is assumed that the HOT lane will always carry its user designated capacity, as long as the adjacent mixed-flow lanes are carrying equal or higher volumes.

Since, the HCM 2010 freeway method does not recognize individual lane capacities, it is necessary to compute an average capacity for freeway sections with HOV lanes, across all lanes.

Equation to compute an average capacity for freeway sections with high-occupancy vehicle (HOV) lanes, across all lanes.

Where:

AveCap(s) = average capacity per lane for section (s) (vph/ln)
CapHOT(s) = capacity per HOT lane (vph/ln)
HOTlanes(s) = Number of HOT lanes in section(s).
CapMFLanes = capacity per mixed-flow lane (vph/ln).
MFLanes(s) = Number of mixed-flow lanes in section(s)

The free-flow speed and speed-flow curve for HOT lanes are assumed to be the same as for mixed-flow lanes, with the only difference being the capacity of the HOV lanes.

HOT Lanes Opened to All

This strategy opens up the HOT lane(s) toll free to all vehicles in the case of a special event, weather event, incident, or work zone. The analysis approach and assumptions are the same as for an HOV lane opened to all vehicles.

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