Every so often, I read a blog post or an article talking about the trade-off between "mobility" and making places more accessible to nonmotorists. The hidden assumption behind such statements is that "mobility" means cars going as fast as possible. So if every street is an eight-lane highway with cars going 70 miles per hour, overall social "mobility" is therefore high.
The use of the word "mobility" to describe fast traffic slants public dialogue in favor of such traffic: after all, who could be against people being mobile?
But we need not define mobility this way. According to one online dictionary I found, "mobility" means (among other things) "the movement of people in a population, as from place to place."*
In a sprawling city where most streets are designed for fast traffic, the mobility of some people (fast drivers) is undoubtedly very high. But the mobility of others is not. In such places, streets are dangerous and uncomfortable for pedestrians- which means that in fact, nondrivers cannot easily move from place to place and are thus not so mobile after all. And even the mobility of drivers is limited: they can be mobile as long as they are driving, but if they choose not to drive for some reason, their mobility disappears.
Thus, government construction of wide, automobile-oriented streets does not create mobility for all. Instead, automobile-dependent places actually eliminate mobility for nondrivers. It logically follows that mobility for all is highest in places that accommodate pedestrians, transit users, and bicyclists as well as drivers- in other words, that accessibility is mobility.