If you want to improve the material conditions of the poor and working classes, what is the one economic metric you should consider most important?
For progressives the answer is income inequality, since a wide disparity between the incomes of the rich and poor is considered by them to be an obvious sign of injustice and a justification for using the force of the government to redistribute wealth. But for conservatives, the answer is upward economic mobility, the ability of an individual or family to improve their economic status. One of the benefits of the free market is that it harnesses liberty, diligence, and hard work in order to advance economic mobility.
The economic realm, though, exists in the physical realm, which is why economic mobility often requires effective means of physical mobility, that is, reliable transportation. While progressives tend to favor government-controlled public transit (such as busses and subways), conservatives tend to prefer individual transportation, especially access to cars. The reason is that history has shown, as Sasha Volokh says, that freedom drives a car: