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I actually don't know on which SE to ask this.

Since there are quite often too low bridges/tunnels, or too high trucks or trailers, why isn't there 100m ahead of those bridges, some light frames (like football goal cage) the size and height of the entrance, which isn't dramatic to destroy, allowing driver to know they won't go through, and giving them time to brake before causing damage to both tunnel/bridge, truck,... ? Or are there already?

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    $\begingroup$ Most countries put signs before the bridge "Low bridge" with the height shown on it.... What's that phrase "you can educate stupid" ... $\endgroup$ – Solar Mike Dec 8 '17 at 12:25
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    $\begingroup$ Yes, these are very common. Usually a big colorful plastic pipe hanging from a beam by chains. Clearance Bar $\endgroup$ – Phil Sweet Dec 8 '17 at 15:11
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    $\begingroup$ I wound up having to design a protective beam for a low rail bridge that was frequently struck despite having a low clearance warning sign. $\endgroup$ – Forward Ed Dec 8 '17 at 15:47
  • $\begingroup$ one of the first jobs I was assigned when I started working at a previous company was to design the anchorage system for a protective steel beam that sat just adjacent to the bridge. The intend was for the beam to take the impact and prevent the rail bridge from being knocked off it bearings or further damaged do its girders. In this case its not far in front of the bridge. However the clearance was posted and low bridge signs were used. $\endgroup$ – Forward Ed Jul 9 '18 at 15:41
  • $\begingroup$ One problem with low bridge warning signs is when a road parallels the tracks and there is an intersection 20 m from the track with the road passing under the bridge. The traffic on the parallel road wont see the low bridge warning sign. $\endgroup$ – Forward Ed Jul 9 '18 at 15:42
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Such measures do exist, but they don't always prevent careless drivers from ignoring the measures and crashing into low bridges.

One bridge that I know of has been hit at least 100 times and has been hit many times since a safety gantry was installed in 2016 to prevent such crashes.

This is a picture of the safety gantry that was installed.

enter image description here

The the bottom of theblack and yellow poles hanging from the gantry swing is the same height above the road as the clearance height of the bridge. When a vehicle that is too high for the bridge passes under the gantry, the poles make a loud noise as the vehicle drives under the gantry, alerting the driver there is a low height hazard ahead.

The bridge is so notorious for being hit, a website has been created which shows pictures of vehicles that have crashed into it and there is a count of days since the bridge was last hit.

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    $\begingroup$ And no doubt they still hit the bridge - as I said you can’t educate stupid... $\endgroup$ – Solar Mike Dec 8 '17 at 19:03
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This web page has a beam just like you are talking about and similar to what I have previously designed for a different site. The difference is, it right in front of the bridge as opposed to 100 m ahead. Nice piece of sacrificial steel. Cheaper to replace it than fix the bridge.

http://11foot8.com/

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