Roads

Induced non-demand

by David Alpert   •   March 11, 2008 9:11 am

Should we really convert freeways to boulevards? In my quick link Saturday about boulevardizing the Southeast-Southwest Freeway, TJ wrote, "the volume day and night is pretty heavy, so a street conversion would just make it a nightmare."

SF's Central Freeway off-ramp recon-
struction. Photo by drain on Flickr.

What's the reality? We can't know for sure about this case, but in other cases where cities have removed freeways and replaced them with boulevards, the volume decreased significantly. Traffic demand is elastic, meaning some people do start carpooling, taking transit, or driving at less congested times of day. But how many? Would a boulevard's capacity suffice for the remaining traffic?

The Preservation Institute has some stats. According to studies they quote, "reducing road capacity does reduce traffic - but not as dramatically as increasing capacity increases traffic." New freeways generate 50-95% new induced demand—half to nearly all of the road's capacity gets filled with traffic that didn't exist before. For removals, usually 14-25% of the road capacity simply disappears when a freeway goes away. When New York's West Side Highway came down, a full 53% of the traffic vanished, thanks to plentiful public transit alternatives.

Would boulevard handle the remaining traffic volume in DC?The analysis is easiest with stub freeways, like San Francisco's Central or Embarcardero Freeways, both of which were replaced with boulevards (though only part of the Central) after damage in the 1989 Loma Prieta Earthquake forced the issue. To take a simple case, the Central freeway's removed portion only carried traffic about four blocks between Market Street and Fell and Oak Streets; as long as the boulevard equaled the capacity of the Fell and Oak intersections at the northern end, the travel time might be slightly greater with a boulevard, but there was no chance of a traffic disaster.

DC's Whitehurst is most similar to this. It's simply a high speed bypass from one crowded intersection at 27th and K to an even more crowded intersection at 36th and M. With almost westbound traffic going to Canal Road in the afternoon backing up well onto the freeway, the real capacity constraint is the traffic light there. We could replace the freeway with a boulevard along M Street and easily move as much traffic as Canal can take off the road on the end.

The Southeast-Southwest Freeway is somewhat of a through route, though as TJ points out, with the Wilson Bridge reconstruction finished much of the through traffic can (and should) take the Beltway instead of cutting through downtown DC. I'm sure most of the commuter traffic coming from Virginia goes to the Capitol or the 395 Central Leg tunnel under the Mall, and likewise for the traffic coming in from Anacostia which may also continue west to L'Enfant Plaza. If that is the case, a new F St SW/Virginia Avenue SE boulevard only needs to handle the capacity needed to get cars to the already constrained side streets.

But we should be able to reduce demand even further. There is a rail line running very close to the freeway for its entire length. If VRE and MARC increased their frequencies and ran trains through from Maryland to L'Enfant and Virginia as I proposed in my fantasy map, many more commuters could shift to trains. We could convert the freight rail line to Anacostia to passenger service.

With many of the commuters going to the innumerable and free Congressional parking lots, parking policy changes by Congress could reduce driving demand and free up land for more Federal employees. While I'm not holding my breath for this, Congress should set up a parking cash-out where, instead of just giving a transit pass or a free parking space, each staffer could receive money equivalent to the amount their parking space costs the Federal government.

And I haven't even gotten into congestion pricing. Between all of these factors, I'm pretty confident that a transportation demand management study would find plenty of ways to cut down on demand enough for a boulevard to comfortably replace the Southwest-Southeast Freeway.

David Alpert is the Founder and Editor-in-Chief of Greater Greater Washington and Greater Greater Education. He worked as a Product Manager for Google for six years and has lived in the Boston, San Francisco, and New York metro areas in addition to Washington, DC. He loves the area which is, in many ways, greater than those others, and wants to see it become even greater.