Comment by dbelford | Hacker News Reader

Comment by dbelford | Hacker News Reader

Parent Full thread

The phenomenon of adding lanes not reducing congestion is generally a reference to the "Downs-Thomson Paradox"(http://en.wikipedia.org/wiki/Downs–Thomson_paradox) or "Triple Convergence" related research.

The general idea is that congestion dissuades some amount of commuters from using the road at congested times. When additional road capacity is added, those people will shift their commute time, or commute mode, or commute route to re-congest the road. (Time, mode or route is the "Triple Convergence" or latent/induced demand.) But in certain contexts like LA or London it's not possible to build enough road capacity to eliminate congestion. So building additional road capacity has the effect of (a) not reducing congestion and (b) reducing usage of public transportation. And (b) causes additional problems for the efficiency that public transportation.

The counter-intuitiveness is why it's a paradox. And the context probably matters for when this theory applies. But supposedly it happens to lots of developed cities. The original study on London was looking at when ~80% of commutes happened by transit. But LA is trying to improve its transit infrastructure because partly because this problem.

The Downs-Thomson Paradox makes reasonable sense, but it doesn't prove the claim from the article. It explains how adding road capacity could make traffic congestion worse (if it causes the preexisting level of mass transit availability to become unsustainable), but not why it inevitably must. For example, if mass transit in a particular city is already useless and disused then that can't happen. It also points to a way out of the paradox: Expand road capacity and subsidize mass transit availability at the same time.

That isn't a reality due to the massive cost of either options (basically, pick one). Successful mass transit using the roads (eg bus rapid transit) require taking capacity out of the highways and therefore aren't mutually compatible.

I had not heard of the Downs-Thomson Paradox, thank you very much for sharing it.

After reading the article I have to point out, from the "Restrictions on validity" section, that it "only applies to regions in which the vast majority of peak-hour commuting is done on rapid transit systems with separate rights of way".

So for most cities outside of NYC, this paradox supposedly does not apply (even though I feel it has some bearing).

Regardless, the general principle behind the paradox is induced demand (http://en.wikipedia.org/wiki/Induced_demand) - which, regardless of the share of transit users, will always apply to highway projects.

Downs-Thomson contains its own ad-absurdum refutation. If adding capacity makes congestion worse, then why can't we make congestion better by reducing capacity? Clearly this is wrong in the general case.

It does not state that it makes it worse, it states that it does not improve it. i.e. that there is an approximate equilibrium that is reached regardless of road capacity in an environment where there is more potential traffic capacity for congested times than the roads could realistically be made to accommodate.

What such simplistic studies miss, though, is the reason why those drivers are out there in the first place. They are going to and from different places, doing different things at different times. By treating traffic as a variable to be optimized in a vacuum, the academic and professional urban planners tend to miss the forest for the trees.

In other words, I believe that to the extent new lane capacity is filled as soon as the construction workers pick up their cones, it is because economically and/or socially useful things are happening. Otherwise, why would any of us drive anywhere at all?

One explanation could be that people are lazy and will generally opt for the conveniences a car if the perceived disadvantages are relatively small. I and many people I know probably would consider a car if we lived in a city optimized for that.

Instead, because the city is optimized for cycling and public transport, I live in a culture where taking even public transport is frowned upon if the bicycle commute is < 30 mins (and even an hour, perhaps).

I think this argument applies to many things. In the city where I live, Amsterdam, there's (almost) always a supermarket at walking distance (or a 5-10 min. bike ride). In fact, this is true for the whole country, as far as I know. 'Hypermarkets' like wal-mart or Carrefoure never really took off. The reason for this is partly culture, but largely it's because of regulation that protects smaller shops.

Most people consider it a very good thing that we can walk to the supermarket and that we are, in effect, 'forced' to have a healthy lifestyle and culture.

Anyways, I just wanted to share how the fact that streets will fill up and hypermarkets will most likely be successful doesn't mean that this is a good thing, or that it's in our best interest.

I think it's a three-part problem. One, it is a flow problem, and by removing congestion at one bottleneck you mostly deliver traffic more efficiently to the next bottleneck. Actually increasing traffic flow is much more expensive than the cost of any single project. Second, over time, you end up with induced demand -- if you actually do get rid of all the congestion, longer commutes will become more practical, and traffic will increase until the marginal-next-commuter is discouraged from adding his car to the scrum.

Third, economic arguments are a little dicey for driving because driving is filled with externalized costs, subsidies, overoptimistic assumptions, and dependent utility. Driving creates noise and pollution (and it appears that the pollution is more deadly than crashes) but drivers don't pay that cost. There's personal crash risk, but people tend to assume that they are careful drivers and hence less likely to crash than the norm. The cost of the roads themselves is currently subsidized from the general fund; it's not a huge external cost, but it's a cost. Driving also creates (perceived) danger to people biking and walking; that tends to encourage them to also drive for their own (perceived) safety, even when they otherwise would not, and the congestion costs of driving also delay bus transit, making it less useful (it's already slower because of all the stops; traffic jams make it slower yet).

Reply on news.ycombinator.com