*Originally posted by Bill H. *
Cite, please. This also flies in the face of common logic. You’re saying that when routes A and B exist, that expanding A will give benefit to B, and expanding B will give benefit to A, but that expanding A will not give benfit to A. I don’t buy it.
Cite, please to back your assertion that lane changes decrease potential flow. Queuing theory (and again common logic) dictates that when a clog occurs, the ability for others to get around it (by changing lanes) increases the flow.
In regards to the above, it’s a fairly well accepted principle of transportation planning called latent demand. Generally speaking, for any road with significant congestion, there’s some percentage of trips that aren’t taken because of that congestion. Hence if you increase capacity, you also increase the trips being taken. This may not be automatic, but any improvement will be relatively short lived, as previously diverted trips discover the new route and fill whatever capacity is created. This may be delayed somewhat by building road capacity far beyond demand, but such excess capacity will soon draw additional development, and you’re right back to where you’ve started with little to no improvement in travel time.
This is a general condition that does not hold in all cases. There are many conditions (for instance bottlenecks) where an improvement could have a very real impact by eliminating a specific impdiment.
With regards to carpool lanes, I’d be rather skeptical of the 50% figure as well. Most cars in the lane will have the minimum necessary number of people in the car, and for that one lane to cary 50% of the people on a four lane road, it’d have to carry more than its fair share of vehicles which is empirically and theoretically dubious.
The real purpose of carpool lanes is to reduce the number of single-passenger cars by providing an incentive to carpoolers. Traffic is a very dynamic system, and one with many externalities. Given that we can’t ever provide enough capacity (see latent demand), the goal is to divert some traffic off the road. One way to do that is to get more people into each car. Thus, the argument goes, encouraging people to carpool serves a societeal good as it helps change people’s habits, and forces single-passenger cars to bear some of the real costs they create.
In reality, most car pool lanes have proven to be politically unpopular, practically useless and difficult to enforce. I’m not aware of any area that is currently building more carpool lanes, and many are taking them away.