SUMMARY:  Studies of the impact of traffic projected for a new development are routinely conducted now, but have not stopped the increase in traffic congestion. This is partly due to the difficulties of predicting human behavior and the cost of doing extensive study. The biggest problem is that such studies only take into account the single development in question and miss the bigger picture. While localities should not stop doing studies of potential traffic impact, expecting them to contribute much to a solution of traffic problems is misguided.

"This session, I will propose a bill to require that rezoning plans are submitted with traffic impact statements so that local officials can get accurate and comprehensive information about the traffic impact of their decisions before they act."

– Governor Tim Kaine in a speech to the General Assembly, January 16, 2006

It might surprise Governor Kaine to know that traffic studies are routinely conducted in support of re-zoning applications in Fairfax County, the jurisdiction with some of the worst traffic problems in the state. Moreover, the use of traffic studies is hardly a new development. Even if we accept the dubious assumption that such studies will provide accurate and comprehensive information to local officials, past evidence has shown that the existence of those studies has done little to keep gridlock from growing.

Why Don't Traffic Studies Always Provide Accurate and Comprehensive Information?

First, traffic studies model somewhat unpredictable future human behavior. Most recently, developers have been proposing to implement traffic demand management (TDM) measures to reduce traffic – such things as shuttle buses, van pools, telecommuting, subsidies to ride mass transit, parking limitation, and others – with the intent of reducing the traffic newly generated by the development so it will have a negligible impact on area traffic. These measures are new, and their impact depends on their changing long-established human behavior. Will commuters and citizens running errands give up their cars? In what proportion? Will employers allow telecommuting and flex time? Will diverting commuters to mass transit overwhelm that transit, driving the commuters back to their cars? Will restriction of parking spaces result in fewer cars in a development, or will the extra cars be parked in surrounding communities? We just don't know, and it's certainly fair to say that most studies take a very optimistic view of TDMs' ability to change established behavior.

It's useful to look at mixed use development in this regard. The re-zoning application is decided long before tenants (stores and businesses) have purchased space or signed leases for the space. Without knowing what stores will be in place, how are we to know how many of their errands the residents can do on site? How do we know how many shoppers will come from off site? If the retail space is a big box store, there will certainly be more traffic generated than if the stores are small service establishments (dry-cleaner, convenience store, etc.). Predictions must be based on a great many unknown factors.

Second, traffic studies are expensive, and they therefore cover the minimum ground necessary. Most studies look in depth at the peak hour of the morning rush and the peak hour of the evening rush. While these studies are certainly better than nothing, current traffic in busy metropolitan areas is heavy 16 or more hours a day, and is often at weekday rush hour levels on weekends. At a minimum, studies must look at a longer morning and evening weekday rush hour and must also study the impact of "errand" traffic on Saturdays and Sundays as well as after-work errands. And they must study the impact of traffic flowing to neighborhood streets in other neighborhoods as drivers discover that it takes less time to cut through existing neighborhoods than it does to use major arteries.

Finally, if these studies are to be paid for by the developer, it is unlikely that the consultant doing the study will discover that the developer's plans will have a major negative impact on the locality. That's not to say that traffic consultants are unethical, but rather that they can present their findings in multiple ways, and their presentation is likely to put the best spin possible on the impact of the developer's plans.

Where's the Big Picture?

The major problem, however, is that a study on a particular proposed development pertains only to that development. The impact may be small or negligible in percentage terms compared to the huge amount of traffic already on major arteries during the peak morning and evening rush hours, especially if we make generous assumptions about the effectiveness of TDMs. But a single development does not exist in a vacuum. Coupled with surrounding new developments, those small percentage impacts may mean a lot of new vehicle trips, and may result in a large total increase in traffic. And it's not fair to ask traffic consultants to calculate the effects of all the proposed developments on the table – developments about which they may find proprietary information inaccessible.

If studies were to investigate entire morning and evening commuting periods, they would perhaps find that rush "hour" becomes longer and longer as commuters choose to go to and come home from work earlier and later than the traditional rush hour. As a result, in Fairfax County rush hour may be said, conservatively, to be from 5:30 AM to 9:30 AM and from 2:30 PM to 7:30 PM. Older drivers trying to avoid major traffic say they have about a two hour window in the middle of the day when they can go out. Again, this is an outcome that studies of the impacts of single development projects have not captured and won't capture.

What's the Role for Traffic Studies?

The good news about traffic studies is that they are quite good for existing traffic. We have an excellent idea of the level of service (A thru F) of existing streets, and we know where the worst problems are.

The best use of traffic studies in the development process would therefore be as part of a phased development. A first phase of building that would not overwhelm existing roads and mass transit would be permitted, but when it was complete there would have to be a pause while traffic studies are conducted to confirm the lack of impact. If that impact is found to be negligible, the next phase of construction would be allowed to begin, and so on until the entire project is complete. You can read about one such plan (that was unfortunately abandoned) at the Forward Fairfax web site.

If traffic studies are to be used as part of the pre-development approval process, they should be paid for by the developer, but done for and supervised by the local government, and the local government would hire the traffic consultant and set the parameters of the study. This would help somewhat, and would allow local residents to have slightly more input into the process via their elected officials.

Conclusion

Mandatory traffic studies sound like a good idea to those not familiar with the current planning and zoning process. It's easy to assume that because traffic problems are getting worse, we're just not doing enough studies now. But we are. Making such studies mandatory is really putting a band aid on arterial bleeding and offering cover to those who will be shocked, shocked, to discover that the more we build, the worse traffic gets – despite what traffic studies for each major development project predict.

Copyright Forward Fairfax and Doug Boulter, 2006