I keep
First aid kit
water
sunscreen
blanket
cereal bars etc
plastic bags
tissues
wipes
coat
books
pens/paper
etc
in the boot, always.
In the winter a whole raft of other things.
My boot always looks pretty full and people take the mic, but I'd rather have less boot space & more 'in case' stuff.
It wouldn't bother me to take a wild wee on the Tarmac. Anything else I'd be able to do in the car if I absolutely couldnt wait a moment more (hence plastic bags, tissues, wipes).
So I'd be 'OK' with an 'open road' 'delay' in my own car, pretty much.
I would not be ok if told to stay in a vehicle, especially not my own. I don't do being locked in.
Tunnel would depend on if I could walk out if I wanted to and just how well I could control my fear of being trapped in.
I have seen emergency services taking vulnerable people out before, but I suppose the difficulty is often where to take them to, so only done in an extreme situation I guess. They have also brought water, formula etc to cars.
It would be bad enough alone/with other ok adults/children, I can't imagine the hell it would be with children (or some adults) with SN or otherwise fragile/vulnerable people. My heart really goes out to them.