This relationship between density and travel times is another counterintuitive puzzler for those who believe cars and roads are the problem, rather than the solution to transit woes. How easy it is to assume that travel times must be shorter where cities are dense and people therefore have shorter distances to travel to work.
What the real world shows us, though, is that when urban population density is lower, and jobs widely dispersed rather than concentrated in a city centre, commuter traffic is more widely scattered on the road network, lowering commuting times. This is a real challenge for the advocates of heavy investment in subways and light rail, which are reliant on commuting patterns focused on a hub-and-spoke model bringing workers downtown along densely developed transit corridors.
I think things are more complicated than that. There are also environmental and social costs associated with greater urban sprawl. Lower population densities lower the economic efficiency of public transport, while higher population densities will by itself increase congestion. It's also much easier to build or expand a new road in environments were there's little or no urban development - imagine trying to build a six lane freeway in the middle of Manhatten.