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7. Updated Recommended Improvement Plan Ramp metering controls the volume of traffic entering a freeway so the system is as effi- cient as possible. As congestion on a freeway increases, the number of vehicles that the freeway can carry decreases. Although a single freeway lane can carry around 2,200 vehi- cles per hour under optimal conditions, as demand exceeds those optimal conditions, the volumes carried actually drop. Under congested conditions, travel lanes have been observed to carry only around 1,600-1,700 vehicles per hour. One source of this congestion is the "turbulence" caused by the merging of vehicles at freeway ramps. By smoothing out this merging, ramp metering can help make the flow of traffic on the freeway lanes more effi- cient and thus increase the volumes and speeds. A survey made for the Federal Highway Administration of seven ramp metering systems in the United States and Canada revealed that average highway speeds increased by 29 percent after installing ramp metering and travel times decreased 16.5 percent. At the same time reductions of freeway congestion av- eraged approximately 60 percent. An analysis of the FLOW system in Seattle (ramp me- tering and HOV lanes) revealed that in addition to similar improvements in speed and travel time, highway throughput increased from 12 to 40 percent as a result of ramp me- tering. An additional benefit from ramp metering is a decrease in the accident rate. Reduc- tions from 20 to 58 percent have been achieved through improved merging operations. Ramp meters can also encourage the peak spreading that needs to occur to keep the gate~ ways flowing. This happens because motorists are willing to accept only up to about a 10~ minute wait at the meters. Beyond that, they will adjust their trip making (i.e., choose to travel at a different time or choose a different mode). This peak spreading helps to get the most out of the system when gateway constraints are a reality. In addition, when combined with HOV bypasses, ramp metering can provide an additional powerful incentive for carpooling and can help buses increase average speeds. When com. billed with HOV lanes on the freeways, the ramp metering-with-bypass system allows car- pools and buses to achieve real travel time advantages compared to single-occupant vehi. cles. Ramp metering has two potential drawbacks: backups on the local street system and re. warding long-distance commuters. The potential for backups on local streets can be mini- mized through ramp widening and strategic placement of the meters. Where these mitiga- tion measures are not possible, ramp metering can significantly reduce levels of service ad- joining intersections and along adjacent streets. The risk of rewarding long-distance com- mutes can be minimized by instituting a system of ramp metering for the entire length of a freeway, rather than in isolated locations. The TVTP/AP supports ramp metering with HOV bypass only where it will not seriously impact local streets and where local implementation is tied with implementation along all of 1-680 and 1-580 in neighboring communities. 18 April 2000 Page 17