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More severe low-tech solution: https://maps.google.co.uk/maps?q=blackwall+tunnel&hl=en&...

(Blackwall Tunnel, northbound, London)

And again, just in case you missed the scraping sounds the first time: https://maps.google.co.uk/maps?q=blackwall+tunnel&hl=en&...

(For some reason they don't appear to have anything similar going south...)

There is a diversionary slip road for overheights (OHT) on the approach to Blackwall after the first pic and before the second. If you've made it to the second (having ignored a good few red lights), expect to be prosecuted.

A fairly low bridge (high enough for a standard trailer, but any oversized loads on a flatbed could easily hit it) in my area had what were essentially large metal cans hanging by wires along a sign warning about the height. Anything tall enough to hit the bridge would hit the bits hanging from the sign and clang them around/together/against the vehicle which makes a pretty audible noise for the driver.

If you hit that sign chances are you'll rip it down and drop it on the car behind you, as can be seen happening in the video in the linked article.