You can also drape a blanket over the stroller or carrier....... and I agree.... you can use the handicap facilities.
I wouldn't recommend leaving them in a vehicle at all...... too big of a chance that someone could break in and take them..........
I could also recommend that you find out where the Petsmarts (or Petco) are on the way. You CAN take them in with you, AND you can use the restroom..... I have even taken pee pads in the restroom with me for them to use!!! ....... I think it is safer than a rest area.