I have an answer.
Presently, all of our emergency vehicles do fit through the underpass. Due to its older design, that also includes the airial truck - just barely. If in the future, we upgrade that truck, newer trucks are generally a little taller.
If there is a train crossing town, or is otherwise blocking all the crossings, and the underpass is blocked or flooded, then the interstate is our only option to get emergency vehicles to the north end of town.