Friday, December 18, 2009

Road Building Paradox

Has anyone ever heard of the Downs - Thompson paradox?
“Downs-Thomson paradox, also referred to as the Pigou-Knight-Downs paradox, states that the equilibrium speed of car traffic on the road network is determined by the average door-to-door speed of equivalent journeys by (rail-based or otherwise segregated) public transport. It follows that increasing road capacity can actually make overall congestion on the road worse."
Sounds like some sort of variation of induced demand.

5 comments:

Winston said...

It's worth noting that the Downs-Thompson paradox only applies on corridors where the majority of travel is done via public transit with separate rights of way. In the U.S. there are precious few places where this is the case. For example in the Bay Area these conditions only apply in San Francisco, across the Bay Bridge (barely) and in bits of Oakland (also barely) which represent maybe 15% of the bay area's population. In most U.S. metro areas these conditions do not occur at all.

The fact is that the best predictor of how congested the road network is in U.S. cities is how much road there is per person. It is in fact the case that more roads = less congestion. This is because U.S. cities have very extensive road networks, aren't very dense and have limited public transit.

The difficulty comes in places like Los Angeles which invested heavily in road expansion in the 1950's-1970's when most of the city was too low density to be effectively served by transit but has grown much denser since then. This higher density means that road expansion is much more expensive and politically difficult and that efficient transit is possible. It is likely that as L.A. continues to densify (and L.A. county added well over 60,000 residents between July of 2008 and July of 2009 despite the recession) transit investment will look better and better. Given this, it is likely that in the case of L.A. their decision in the 1990's to focus on transit was the right one.

MB94128 said...

For those who are wondering, Los Angeles fell victim to the transit rape of the mid-twentieth century. Had they stuck with rail-based transit and invested in it with less emphasis on freeways there would probably be a lot more riders than there are today.

Metro (L.A.County Trans.Auth.)
Los Angeles Transit History
Los Angeles Transportation History Archive

Other :
Los Angeles Rwy. (map @ 60%)
Pacific Electric
P.E. system map

Anonymous said...

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M1EK said...

Peter, are you aware that none of the major central employment centers are located within walking distance of a station on this line? (Even most downtown office buildings are 1/2 mile away or more). Why do you think it'll be a success when the only other commuter rail line with these characteristics (Tri-Rail) was an unmitigated failure?

Pantograph Trolleypole said...

M1ek I think you meant to comment on the other thread.