Generations? The average American passenger vehicle is 14 years old, so if tomorrow all new cars were electric, you would have haved car transport emissions within 15 years, and be at a 75 percent reduction within the first generation. Cut out fossil fuel subsidies so people are paying the 8 or so dollars per gallon it actually costs for gas and incentivize US manufacturers to actually build affordable cars and you’ll see much quicker adoption that what normal wear and tear causes.

Of course that isn’t going to happen tomorrow in the US, but you are also going to have a lot of vehicles already sold in the decades prior and which tend to stay on the road longer.

Compared to the fifteen or so years it takes to build a single light rail line, much less intercity high speed rail, and you are not going to be able to replace half of all car traffic in a single build cycle, much less reach 75 percent within thirty years, by which point you’re trying to replace all traffic in the very small towns and unincorporated areas that even nations renowned the world over for their public transit connectivity often struggle to reach.

Does the US need to build more mass transit, yes. Can it do so faster than it already buys new cars, no.