I think a corridor sounds great. and just err on the side of giving it time to be wrong (say 400 meters out of the corridor). then before reroute the voice could say "vehicle possibly off course, rerouting in ten seconds, doubletap app to disable rerout and contine using current route" (or something to that effect).

That way a reroute after it being detected you are a reasonable way off course would be the default, however the user would have a simple way (double tap screen within 10 seconds of the message) to disable the reroute. the message could even be "rerouting in 10 double click to disable".