if you're referring to Emergency Advisory Radio/Travelers Information Radio, those are low-power stations usually found on 530-khz or brtween 1600-1710 khz. they can be heard usually only within a few miles of the transmitter and are usually identified by a sign like this:

if you're referring to a highway sign listing area AM and FM stations for weather info...

those are commercial radio stations which, assuming no technical issues are keeping them off the air, should be able to be heard. some AM stations are 1000-watt daytime only stations, others are 5K-watts and the big blow torches are 50KW.