Talk:Downs–Thomson paradox

Help needed to point out the differences, if any, between the different names - Downs-Thomson paradox , Pigou-Knight-Downs paradox, Braess paradox and Lewis-Mogridge Position.Mario J Alves 22:29, 18 August 2006 (UTC)

Presumably the opposite also happens: slow down road transport, people shift to rapid transport, and that slows down too. --Henrygb 23:07, 16 February 2007 (UTC)

Comment on "Presumably the opposite ..."

"The opposite also happens" in the SHORT RUN,  but in the LONG RUN  shifting people to rapid transport systems leads to increasing frequency of service, which reduces waiting time, making the total transit trip a little quicker and more convenient. As a result, more people will shift to rapid transit, and more service wll be added,  making the rapid transit still quicker and still more convenient, and making the competing road network ALSO quicker and more convenient.  Eventually the rapid transit gets used enough so that the exception mentioned in the text no longer applies.
 Conclusions
    Good public policy: add rapid transit and reassign more lanes to car-pools.
    Bad public policy:  add capacity to road network while negecting rapid transit.  —Preceding unsigned comment added by 76.176.188.224 (talk) 21:47, 24 August 2010 (UTC) 

Restrictions on validity

Here is a more complete quote from pp.132-133 of Downs' Still Stuck in Traffic:

Transit Capacity Could Reduce Intense Traffic Congestion in One Situation

In theory, there is one exception to the conclusion that expanding transit capacity cannot reduce existing intensive peak-hour traffic congestion. It was pointed out by Martin J. H. Mogridge. However, his analysis only applies to regions regions in which the vast majority of peak-hour commuting is done on rapid transit systems with separate rights of way. Central London is an example, since in 2001 around 85 percent of all morning peak-period commuters into that area used public transport (including 77 percent on separate rights of way) and only 11 percent used private cars. When peak-hour travel equilibrium has been reached between the subway system and the major commuting roads, then the travel time required for any given trip is roughly equal on both modes.

That must be the case because travelers can shift modes between transit and roads. If movement on the roads generally takes less time, then people will move from the transit system onto the roads. The added road traffic will slow the average speed on the roads--that is, increase travel times for specific journeys. So roads cannot maintain a travel time superiority over transit very long. Even expanding road capacity does not speed up movement on roads permanently. Rather, doing so simply draws more passengers from transit until the roads become so loaded they slow to the same travel time as transit. Since the percentage of travelers using transit is so great, there are always enough transit passengers willing to shift onto roads to swamp any improvement in speed from building more or wider roads. That is what differentiates this situation from conditions prevailing in every American city except perhaps New York City.

However, if travel time (including getting to the station, waiting, and getting from the station at the other end to the final destination) is less prolonged on transit than on roads, people will move from roads onto the transit system. But unlike roads, fixed-rail transit systems with separate rights of way do not become slower because they have more passengers--they only become more crowded.

...

Mogridge concluded that, in the long run, the speed of movement on the transit system--that is, its travel times--is the key factor deciding the overall speed of all modes in the city.

So I think the entire section called "Restrictions on validity" should be removed. Traal (talk) 00:29, 26 September 2022 (UTC)

I had to read the section several times, before I realized that the restriction on validity applies to the consequences, not the paradox itself, so if anything, the section should be moved to a subsection of consequences. I would perhaps argue that the text doesn't require a subsection by itself at all and that it should be stated directly under consequences? - If these changes are done, does the text still seem unreasonable to you? BFG (talk) 11:23, 26 September 2022 (UTC)

I believe that the exception described in the passage above is to the general rule that adding transit capacity doesn't reduce car traffic congestion. But that rule isn't mentioned in the article, so before the exception can be explained, the rule must be explained first, and how it's related to the Downs-Thomson Paradox. Traal (talk) 14:24, 28 September 2022 (UTC)

I was not aware, but according to [1], what is styled as the consequence in this article is what was originally styled as the "Iron Law of Congestion" but here that is referred to as the "consequence" of the paradox. I don't have access to the original book by Downs, but it seems appropriate to mention the "Iron Law of Congestion". It may even be appropriate to restyle the article as "Iron Law of Congestion" and talk about the paradox as part of the article, with a redirect from the paradox. BFG (talk) 13:46, 30 September 2022 (UTC)

[2] provides a summary: "'We know from experience, in Texas city after city, that a simple addition of lanes will not fix our traffic woes,' wrote Austin city council member Gregorio Casar, in a letter to TxDOT. There's a name for the principle behind that apparent paradox: induced demand. Economist Anthony Downs is often credited with first articulating this 'iron law of congestion' in 1962, as construction crews were hacking interstates through American cities. Downs published a seminal paper with a stark warning: 'On urban commuter expressways, peak-hour traffic congestion rises to meet maximum capacity.' In other words, adding lanes won't cure snarled traffic; the additional car space inevitably invites more trips, until gridlock is as bad as ever." So it seems we're talking about two different but related paradoxes, induced demand (adding lanes won't reduce traffic congestion and may even increase it) and Downs-Thomson (traffic speeds are set by the speed of public transport). Traal (talk) 14:33, 1 October 2022 (UTC)

Currently, the Restrictions read as though the Downs-Thomson Paradox only applies when there's significant public transport infrastructure already in place. Which runs contrary to the above passage from Still Stuck in Traffic which states that the only time improving public infrastructure doesn't improve transit times is effectively when nearly everyone is already using it anyway (ie when it's already optimized compared to cars). If public transport isn't optimized, then improving it still improves door-to-door speed of the public transport journey, which results in improved car based journeys due to the offloading of the car network onto the public transit network. — Preceding unsigned comment added by 203.51.194.18 (talk) 07:21, 12 October 2022 (UTC)