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.In many cases, rampmetering has been implemented as part of a suite of technology-enabledtraffic-management strategies (Papacostas and Prevedouros, 2001).Evaluation of StrategyCost/Revenue ImplicationsRating: Medium cost.The costs of ramp-metering systems varydepending on the level of technology employed as well as the scopeof implementation, increasing significantly with the addition of com-munication networks, surveillance systems, and centralized computingrequired for SWARM systems.Several studies have found that precisetiming is critical for achieving and maintaining freeway flows (Hellingaand Van Aerde, 1995) and that, under certain circumstances, impropertiming can negate any benefits of metering (Banks, 1988).This under-scores the importance of ongoing expenditures related to operations,maintenance, and other associated costs. Freeway-Ramp Metering 199An evaluation of ramp metering in the Twin Cities area spon-sored by the Minnesota Department of Transportation (MNDOTet al., 2001) estimated system capital costs on the order of $63 mil-lion and annual operating costs of $1.5 million for a system of 430ramp meters, 3,500 traffic sensors, 230 closed-circuit television(CCTV) monitors, and a traffic-management center (TMC).Note,however, that the CCTV surveillance system and TMC were the twohighest-cost items, and both of these elements can support other traf-fic-management strategies, such as incident-management systems andadvanced traveler-information systems.As such, these high costs maybe spread across multiple programs, and the actual percent attributedto ramp-metering systems will likely differ from case to case, depend-ing on agency-specific cost-allocation protocols.Short-Term Effectiveness in Reducing CongestionRating: Medium (uncertain).Ramp metering can be effective inmanaging congestion, both by increasing aggregate vehicular through-put and capacity and by reducing crash rates on the freeway main lines.Several studies have documented flow improvements and delay reduc-tions with the use of ramp metering.In an evaluation of the MinnesotaDepartment of Transportation s ramp-metering program in the TwinCities, for example, it was demonstrated that the technology increasedfreeway throughput by 9 percent on average and by 14 percent duringpeak periods.This translated to more than 25,000 hours of annualtravel-time savings for the region (MNDOT et al., 2001).Anotherstudy of Denver s ramp-metering system found that speeds increasedup to 58 percent when the meters were turned on (Corcoran and Hick-man, 1989).Such dramatic improvements, however, are not certain.A simu-lation of ramp metering in Orange County, California, for example,showed that significant levels of traffic had to be diverted to arterialstreets in order to maintain efficient flows on the freeway network andthat improvements were negligible if parallel streets were already oper-ating at or above capacity (Nsour et al., 1992).Another study reachedsimilar conclusions, finding that passenger hours dropped by 6.3 per-cent in cases in which there was unused capacity on alternative streets 200 Moving Los Angeles: Short-Term Policy Options for Improving Transportationbut actually increased by 3.1 percent in cases in which the parallel arte-rials were operating at capacity (Alexiadis and Schmidt, 1994).Because ramp-metering effectiveness can depend on arterial-street conditions, we rate this strategy as having medium effectivenessin short-term congestion reduction but also describe the rating as beinguncertain.Long-Term Effectiveness in Reducing CongestionRating: Low.Ramp metering is intended to reduce delays on thefreeways.To the extent that it is effective in this regard, however, itsbenefits will tend to be undermined in the longer term based on thephenomenon of triple convergence.Moreover, ramp metering will dolittle to stem longer-term increases in congestion resulting from contin-ued population and economic growth.Mobility, Accessibility, and Traveler ChoiceRating: Good.Ramp metering, if successfully implemented, willenhance mobility by increasing the freeway network s effective capac-ity.Note, however, that ramp metering improves freeway performanceby managing the rate of vehicles using the on-ramps.If this causestraffic to back up onto the arterial system, traffic on the surroundingsurface streets may be impaired.In addition, ramp meters may have alimiting effect on traveler choice by diverting short trips off of the free-way and onto local streets (Papacostas and Prevedouros, 2001).Despitethese problems, improved freeway flows the benefits of which accrueto automobiles as well as any trucks and buses using the freeways canprovide increased mobility across the region.For this reason, we rateramp metering as having good effects in this category.SafetyRating: Very good (uncertain).Several studies have shown thatramp metering can lead to significant improvements in vehicularsafety, especially in terms of reductions in rear-end and sideswipe col-lisions (Caltrans District 7, 2007) [ Pobierz całość w formacie PDF ] |
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