my first thought actually was, how would the cars react to road spikes thrown by police? Especially on a highway with barriers on either side.

Of course a competent writer would've thrown in a line about how these cars are on run flats at some point...

Our only hope is for the scientists in the So-Secret-President-Doesnt-Even-Know Facility to come up with something so crazy it just might work

An extremely selective and directional EMP cannon springs to mind as being sufficiently Hollywood

There are actually already physical barriers throughout the governmental parts of Washington DC to prevent this sort of thing. There are permanent walls, blocks, and poles along the edges of the roads (often well-integrated into the architecture,) and raise-able barriers built into the road surface at intersections. Ain't no cars running over our president!