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Executive Summary

Traffic congestion and delay have become an endemic component of commuting life in the Washington DC region. To many, the unpredictability of travel time is almost more annoying than delay—one day a 10 mile trip may require 20 minutes, and the next day 45 minutes. Because the system is so near capacity, and exceeding capacity in some areas, a minor incident or a rainstorm, or simply too much traffic, causes major breakdowns and systemic delays. In this research study we demonstrate that there is a way to restore reliability and predictability to our highway system, without spending billions on new lanes of traffic.

Traffic congestion in the Washington, DC area, especially congestion on our freeways, costs our residents every day in terms of wasted time, fuel, and increased air pollution, including green house gases that are a primary cause of climate change. Highway studies have determined that once traffic volumes exceed the capacity of the roadway, the system can rapidly "break down" to the point where all traffic slows markedly, and the capacity and throughput of the roadway drops precipitously. The Federal Highway Administration commissioned this study to specifically evaluate congested versus uncongested travel on some of the major roadways in the metropolitan Washington region, to identify the specific "tipping point(s)" at which free-flow traffic "breaks down", and conversely, the volume of traffic that would have to be reduced in peak periods to keep traffic free-flowing. The study also examined travel behavior based on the Metropolitan Washington Council of Governments Household Travel Survey, to estimate the number and percent of trips that people take in peak hours on our freeways that are discretionary trips. With appropriate incentives or disincentives, many of these discretionary trips could be shifted to off-peak hours or otherwise deferred. Finally, the study reviewed empirical findings on experiences with congestion pricing in the US and abroad, to provide ranges of estimates of the amount or percentage of traffic that could be shifted out of the peak period or encouraged to use ride-sharing through a comprehensive pricing and transportation demand management program.

The traffic analysis (Section 2) focused on a 12.9 mile segment of I-270, a 10.5 mile segment of I-95, and two independent count locations on the Capital Beltway for the evening rush hours (PM Peak).[1] The study compared speeds and volumes in the summer and on holidays to dates and times in September and October, when the system can be observed to "break down". This pair of figures is one of many examples—the "Volume-Speed Relationship at Selected Chart Detectors: I-95 Northbound on Wed 8-1-07" figure shows volumes (horizontal axis) and speeds (vertical axis) on August 1, the "Volume-Speed Relationship at Selected Chart Detectors: I-95 Northbound on Thu 11-10-07" figure shows October 11, when a short "spike" in demand generates an extensive period of lower volumes and lower speeds.

Line graph for volume-speed relationship for I-95 Northbound on August 1, 2007


Line graph for volume-speed relationships for I-95 Northbound on October 11, 2007

A main finding of this study has been that if relatively small changes can be made in peak demand (volume) through various programs and strategies, such as congestion pricing, then two beneficial things can happen: (1) there can be relatively large decreases in congestion and delay, especially at key choke points, and (2) there can be increased through-put along those roads during peak times of travel — thus by effectively managing the demand more travelers can be served per time period with the available fixed-supply of roadway capacity.

The analysis has shown that in many instances the amount of needed demand reduction can be on the order of five to ten percent of the peak period flow. However, there still may be a particularly difficult bottleneck in a corridor that would need reductions on the order of 15 to 20 percent. While demand reduction and operational strategies may be able to go a long way in improving the flow at those locations so that capacity is exceeded less often and/or recovery is quicker, there still may need to be localized geometric or lane use changes at those locations to routinely have freer flowing traffic at those troublesome locations.

We established that in general a 10 to 14 percent decrease in traffic on congested freeways will reduce delay by approximately 75 to 80 percent. For example, on I-95, an average traveler during the peak period would save 110 seconds each day, or about 5.3 cents per vehicle mile for that 10.5 mile stretch of road. During the most congested peak hour, the average savings in delay would be 310 seconds per person per day, or about 14.6 cents per vehicle mile. On I-270, the average traveler would save 220 seconds during the full peak period, or about 8.5 cents per vehicle mile. During the peak hour on I-270, a traveler would save about 340 seconds for that almost 13 mile stretch, or about 13.2 cents per vehicle mile. Those savings per traveler, multiplied by all drivers, multiplied by 250 work days, yields millions of dollars in annual time savings from reduced delay for these two roadways alone.

In Section 3 we establish that from 7.7 percent (AM Peak) to 10.5 percent (PM Peak) of the longer trips in personal vehicles (PV) are typically discretionary. The definition of discretionary trips includes restaurant visits, family/personal trips, shopping trips and social/recreational trips. Discretionary trips, by definition, should be fairly easy to divert to non-peak times or to routes other than freeways. We do not expect to divert all discretionary trips, but the 7 to 10 percent gives us a substantial base to start from, as we aim for a 10 to 14 percent reduction.

Pie graph depicting total versus discretionary AM peak trips


Pie graph depicting total versus discretionary PM peak trips

In Section 4 we demonstrate that modest pricing signals for private vehicles can reduce traffic enough to significantly reduce congestion and save time for all drivers, while at the same time increasing the "people-carrying capacity" of the roadway. Experiences across the country and around the world have found that charging a modest toll for single-occupant vehicles while improving the availability of carpools, vanpools and transit can create significant shifts in travel behavior, in the ranges necessary for the DC area.

It therefore appears feasible to restore and maintain free-flow on the freeways in the Metropolitan Washington area, without adding capacity (except to alleviate selected bottlenecks), by applying congestion pricing to the major facilities, and at the same time increasing transit, carpool and vanpool programs. The combination of diverting most discretionary trips to other times and diverting an additional five to ten percent of personal vehicle work trips to HOV modes should achieve the needed 10 to 14 percent overall decrease in traffic needed to achieve major reductions in delay.

Such a system would require phased implementation and experimentation to identify workable technologies and appropriate rates to achieve the desired result.[2] (Implementation strategies are beyond the scope of this study, but many different proposals have been evaluated for the Washington region.) Rates could be adjusted up or down, to ensure the roadway is used to near-maximum capacity, without exceeding capacity to the point of breaking down and failing. Revenues collected could be used to improve HOV alternatives as well as maintain the roadways, address choke points and bottlenecks, and improve alternate routes. Finally, travelers in the region would truly benefit and travel in confidence, knowing that they can reliably predict their travel time on a daily basis.




[1] The analysis also examined volume and speed for two segments in the morning rush hour (AM Peak).

[2] It is significant to note that the MWCOG Climate Change Report, approved by the COG Board November 12, 2008, mentions both evaluation of financial incentives such as congestion pricing and incentives for expanded transit use to reduce Vehicle Miles Traveled (VMT) and greenhouse gases (page 61). www.mwcog.org/publications.

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