What to do if you miss last U-turn exit before crossing the border (youtube.com)
People miss or ignore the signs and panic.
‘It would probably be good instead of having just one sign, to have two more.’
How about a big sign that says ‘This road goes into Mexico in 5 miles. There are only 2 more exits and then you will be forced to drive into Mexico before returning. Take an exit immediately if you don't intend to enter Mexico.’ and this should also be programmed into Google maps directions, Waze, etc.