The safest speed is slightly above the posted speed limit. A maximum speed of 60 MPH is not safe on a 70, 75, 80 MPH highway. Driving slow makes people behind you impatient and angry and thus you will do things you might not usually do in your rush to pass. These vehicles need to be able to drive highway speeds.
Other than that, I like this. If enough recharging stations are available, then the 30 minute wait for trucks isn't a big deal like it is for passenger vehicles. Truckers need 30 minute breaks to stretch the legs and to help prevent drowsiness.