These extraordinary...

These extraordinary pictures from the Berkshire-based Transport Research Laboratory (TRL) prove that the weight of the variable message signs brings the entire gantry collapsing on to the road when a support leg is struck by a car.

Highways engineers are keen to use the new lightweight lattice beams as they are cheaper. But every object placed on motorways has to go through rigorous safety checks first - and this one failed.

Scientists know from earlier tests that the lattice beams absorb impacts better when used for supporting road signs. But the full-scale sign gantry crash test - carried out in controlled conditions at a TRL test centre - was an eye-opener.

"Our Rover 75 was doing 70mph when it struck the gantry leg," Dr Tim Gammon, of TRL, explained. "Occupants would have survived the crash because the fragile design of the support took a lot of energy out of the impact."

That part of the exercise was a success, but it would have been bad news for other road users had the crash happened in real-world conditions. The conventional variable message sign, which weighs 830kg, brought the whole structure to the ground, creating a hazard. To see if the problem could be solved, boffins conducted a second experiment with a 430kg sign. This time, the gantry stayed upright when it was hit by an identical car at the same speed.

Now, there are calls for lighter signs to be made to boost road safety. The M42 in the West Midlands has heavyweight signs on solid gantries every 500 metres, and the Highways Agency is assessing them. Transport experts hope the safer lattice gantries could still be used if new, lighter signs get approval.