Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Appendices
provide information to drivers about parking availability or allow for reservations
would address this issue.
5.4 Traffic Demand Metering
Traffic Demand Metering strategies aim at reducing the probability that a freeway or major
roadway will break down by controlling the rate and location of additional new demand (i.e., from
on-ramps, toll plazas, etc.). The metered traffic is allowed to enter the freeway or major road at a
rate that is compatible with continuous or “sustained service flow” on the mainline. The objective
of traffic demand metering treatments is to smooth out demand to better match the available
capacity on the freeway and thereby significantly improve freeway performance.
Metering can also improve travel-time reliability. An experiment where ramp meters were shut
down for a six-week period in Minneapolis-St Paul during 2000 indicated that travel times were
nearly twice as predictable when the meters were on, as compared to the off condition (6).
Traffic demand metering treatments consist of mainline metering, on-ramp metering, and peakperiod
ramp closures.
Ramp Metering, Ramp Closure. Freeway on-ramp metering can be operated in different
schemes to control the manner and rate at which vehicles are allowed to enter a freeway. Onevehicle-per-green
metering or a tandem metering scheme with two entry lanes are usually
implemented. Since the first ramp meters in the United States were installed in the 1960s,
hundreds of installations have been made with favorable operating results. Included are a number
of “mainline metering” (usually implemented in bottlenecks caused by bridges or tunnels) sites,
such as those on Oregon Route 217, San Diego I-8, and Los Angeles I-710 (6). Postimplementation
studies of peak-period freeway ramp metering have shown very little to no
diversion of demand to downstream freeway ramp sections.
In Honolulu, Hawaii, as an experiment, the Lunalilo Street entrance ramp on the H-1 freeway was
closed during the a.m. peak period for two weeks. This closure resulted in 10 minutes of traveltime
savings along the H-1 freeway. This experiment was followed by a pilot project to close the
entrance ramp from 6:00 a.m. to 9:30 a.m. The ramp closure was made permanent in the fall of
2004 (14).
A study of the integrated deployment of freeway ramp metering and adaptive signal control on
adjacent arterial routes in Glasgow, Scotland, found a 20% increase in vehicle throughput on the
arterials and a 6% increase on freeways. Arterial traffic flows increased 13% after implementation
of ramp metering and an additional 7% with the initiation of adaptive signal control (9).
5.5 Variable Speed Limits
A variable speed limit strategy dynamically adjusts the facility speed limit by lane according to the
facility operating conditions. As a facility approaches capacity at a bottleneck, the speed limit for
upstream sections is reduced to decrease the shock at the bottleneck. In many applications, variable
speed limits are embedded in traffic control systems that also adopt other measures such as lane
control, ramp metering, or temporary hard shoulder running. The application of variable speed
limits increases road safety by displaying traffic-adaptive speed limits as well as warnings in case
of incidents, traffic congestion and road blockage or bad weather conditions. It also helps
homogenize traffic flow through work zones (5, 6)..
Variable speed limits (speed harmonization) are widely used in a number of European countries,
particularly on freeway sections in metropolitan areas with large traffic volumes where traffic-
ADDITIONAL DESCRIPTION AND QUANTITATIVE BENEFITS OF TRAVEL-TIME RELIABILITY STRATEGIES Page F-11