"When the headroom of an arch or span of a bridge is reduced from its usual limits but that arch or span is not closed to navigation, the person in control of the bridge must suspend from the centre of that arch or span by day a bundle of straw large enough to be conspicuous and by night a white light."
Does that mean the law is not being complied with, in this case, since the bales are hanging from adjacent bridges, not the "centre of that arch or span" itself?
Perhaps we should try a bale of straw next.
The London Blackwall tunnel has a more modern take on checking height: https://maps.app.goo.gl/b5P5Td1hsuSjLU3w8 traffic signals, barriers like at a railroad crossing, giant panels across the road at height, and a police car on standby to pull out and fine anyone that doesn't read the signs - I presume this happens often enough that they can justify the cost.
But then the bale of straw applied to ships not vehicles and bridges not tunnels.
It doesn't surprise me too much that police are on standby, a closure of either tunnel or bridge has a major effect on traffic all over London
I'd have a feeling there are automated signs prior to the tunnel (or at least used to be) but I've not been through the tunnel for a year or so and things will have changed with the Silvertown tunnel opening.
I have seen someone not paying attention at the Rotherhithe tunnel and the roof of their van was a mess (and they're going to pick up a fine probably due to restrictions, the 2 tonnes gross weight limit is lower than a lot of van drivers expect)
Edit there were: https://maps.app.goo.gl/MP7fkhS394DJPQaZ9 If you zoom in you can see the overheight vehicle warning.
I question who approved that the main lanes ahead of your link have 2.8m/9ft limits but the police warning says vehicles over 4m/13ft will be stopped. Can I take my 10ft truck through or not?
I'm starting to feel a tiny bit of sympathy for drivers that get confused by this.
Blackwall seems also to have two sets of lights and barriers, and an off-ramp in between. That's probably for fire safety too to close and evacuate the tunnel and get the emergency services in, but I imagine it's used for height detection too if a loud CLUNK on your truck cabin isn't enough.
As an aside, the person who signed the original heights as (13ft)(4m)(9ft)(2.8m) needs to learn a bit about UI design. Yes, two lanes, but the gap between the central two signs is far smaller than to the other sign for the same lane. Also 4m is just over 13 ft 1 inch, which there'd be space to include as there's already a 0 on the leftmost sign (and from the rightmost we see that decimals are allowed on signs). Guess we're going to rely on the CLUNK after all.
I also imagine it wasn't cheap doing this, but apparently as long as people can get away with something there is always those that will try, regardless of how it impacts others.