The SS can and does make that information available when necessary, for instance to private toll bridge operators. Waymo doesn't need significant advance notice either, just minutes for normal vehicles.

I suspect the easiest thing would be a "return to base and shutdown" button that the Secret Service could trigger at short notice - would cause problems for everything wanting to use a Waymo, but I suspect the SS are quite OK with causing disruption already.

Sounds like fake movie spy shit to me. It turns out, the Secret Service is an organization of humans with ordinary lives and a relatively modest budget.