Lots of great replies. Thanks.
My fundamental problem is that if the barrier is too low, then by the time it scrapes the roof then it is too late, it has already cost me a respray and a family argument.
It's a rainy day here, so I've been thinking about an early warning solution that's less entertaining for any audience, and more elegant than standing on the step or hand signaling furiously with my wife!
Here's a low tech idea:- Attach a pipe to the roof rails, pointing lengthways down the van. That then becomes the highest point by 5mm or so.
Something like this or maybe a bit narrower.
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I can have it adjustable so that it can extend over the bonnet a little bit and I can see it from the comfort of the driver seat. If it touches the barrier then only the pipe gets damaged. If the barrier is just a hanging bar, then the pipe can push the bar out of the way!
Here's a higher tech solution from another thread. Except the camera could be mounted on the roof rail somewhere. An electronic wife-on-the-roof if you like.
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Yes, yes, I know I'm studiously avoiding the obvious tape measure solution, but I just don't want to get out of the van in the rain and measure the barrier