I admit that title is a little strong, but “How HOV lanes probably contribute to pollution” just doesn’t grab you the same way. Anyway, as I’ll get to shortly it’s an empirical matter that I think can be reasonably inferred to swing the way I think it does.
Fact: cars in stop and go traffic emit more pollutants than cars driving at more even speeds. HOV lanes, in effect, cut off lanes for most vehicles. The idea was to encourage people to carpool, which may be admirable. However, many of the people who use the HOV lanes are people who would already be in vehicles together. Parents with children, for example. This probably varies from city to city. The situation that made me think of this was in Alexandria, Virginia, in the Washington D.C. metro area. There is a high percentage of childless professionals here without strong social roots, so that it’s far less likely that the HOV lane will be in use than in other areas of similar size and build.
When there is heavy traffic in other lanes and the HOV lane is either largely empty or has traffic moving at a much faster clip, this implies that traffic could be eased in the other lanes if it were not for the artificial legal restriction. Traffic at any speed produces pollutants, but traffic at lower speeds produces more, such that in the cases I’ve described more pollutants result. What about when traffic in the HOV lane is moving at the same speed as traffic in the other lanes? Well, in that best case scenario, the HOV lane is not providing any special benefit to carpoolers; in effect, it’s just another lane.
The first objection to what I’ve just said is that it is supposed to encourage people to carpool. That is an empirical claim, but as far as I can tell it really doesn’t. The second objection is, I think, a bit stronger: maybe the added traffic hassle will cause marginal commuters to work from home, take public transit, or some other way of staying off the road entirely. That would cut down on pollution. This is also an empirical claim, and it’s hard to measure. I don’t expect that it really leads people to stay off the road who weren’t already incentivized to do so.
I wrote the previous section before looking up any data. I wanted to challenge myself to think it through as much as possible. After a quick look at the available research, I appear to be on the right track. Here’s one example.