Traffic congestion on a freeway sets in once the density of vehicles exceeds a certain critical number. Above that, both vehicle speed and vehicle ﬂow drop precipitously. Well-designed ramp meters can limit the number of vehicles entering a freeway, so that critical density is not reached, congestion is avoided and, paradoxically, both speed and ﬂow increase. This double gain of reduced travel time and increased ﬂow far exceeds any improvements that can be achieved by constructing more freeway lanes.
Figure 1 shows the cause and consequence of congestion. It plots speed vs. ﬂow on the fast lane on one section of west- bound I-10 in Los Angeles from 4:00 a.m. to noon. Until 5:10 a.m. there is no congestion—a ﬂow of 2,100 vehicles per hour (vph) moves at 58 mph. By 5:30 a.m., the density doubles, causing congestion, and speed drops to 30 mph. At 7:00 a.m., speed is a stop-and-go 15 mph, and the ﬂow decreases to 1,300 vph. Only at 11:30 a.m. has demand and the resulting density decreased sufficiently to restore the free-flow speed of 60 mph. In the depth of congestion at 7:00 a.m., efficiency of this section is 15 percent, down from 100 percent at 5:00 a.m.
Figure 2 gives the macroscopic picture for all of Los Angeles. We examine data from all 3,363 functioning detectors at 1,324 freeway sections in LA for the 12-hour period beginning at midnight on September 1, 2000.
For each detector we find the 5-minute interval during which the detector records maximum flow. We then find the average speed at each detector during the 12.5 minutes before 12.5 after this maximum-ﬂow interval. The ﬁgure plots the distribution of this “speed at maximum ﬂow” for each lane. Clearly, maximum ﬂow occurs at free-ﬂow speeds ranging from 65 mph in lane 1 to 55 mph in lane 4. These data show that:
- The most efficient freeway operation occurs when traffic is moving freely at 60 mph and not at 35 to 45 mph as is commonly assumed.
- Ramp metering, which controls vehicle entry so that traffic moves freely, will reduce travel time and increase ﬂow, permitting optimum efficiency of the freeway.