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

Appendix I: Speed/Capacity for Ramp Metering Strategies

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

The 2010 HCM freeway operations analysis method is not sensitive to the effect of ramp metering on the capacity of merge sections. The coded capacity of the freeway merge section is therefore increased by 3% for those days, hours, and locations where there is ramp metering in operation (source for capacity effect of ramp metering: FHWA Ramp Management and Control Handbook, January 2006).

Locally Dynamic

For locally dynamic ramp metering an adaptation of the ALINEA (M. Papageorgiou, H. Hadi Salem, and J-M. Blosseville (1991) “ALINEA: A Local Feedback Control Law for On-Ramp Metering,” Transportation Research Record, vol. 1320, pp. 58-64) algorithm is used to estimate the ramp metering rate for each analysis period for each scenario.

Equation 9. For locally dynamic ramp metering an adaptation of the ALINEA algorithm is used to estimate the ramp metering rate for each analysis period for each scenario.

Equation 9

Where:

R(t) = ramp metering rate for analysis period (t) (vph/ln)
NR = number of metered lanes on ramp.
CM = capacity of downstream section (vph)
VM(t) = volume on upstream section for analysis period (t) (vph)
VR(t) = volume on ramp during analysis period (t) (vph)
QR(t-1) = queue on ramp at end of previous analysis period (t-1) (veh)
QRS = queue storage capacity of ramp (veh)
MinRate = user defined, default value is 240 vph/lane
MaxRate = user defined, default value is 900 vph/lane

The current version of the HCM 2010 freeway analysis procedure does not include ramp storage lengths, so there is no constraint on the estimated metering rates related to ramp queue storage lengths. Should such a capability become available in the future, then the following additional constraint would be applied to the above equation:

Equation 10. Equation to estimated metering rates related to ramp queue storage lengths.

Equation 10

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