Cattle grids confusing some modern cars, because emergency braking sensors mistake them for obstructions

Cattle grids are confusing some modern cars and causing them to crash, because emergency braking sensors mistake them for obstructions, a council has said.

Urgent work has been carried out in Somerset after the council warned that a grid was “causing a very real danger to road users” and blamed modern technology on new vehicles.

The grid on Hill Road in Minehead was cut into the hill in a step which was causing onboard computers on some more modern vehicles to apply advanced braking systems for what appeared to be an obstruction. 

This resulted in a number of incidents with cars leaving the road, the council said.

“Thankfully there have been no serious collisions but something needed to be done,” they added.

This Cattle grid caused some cars to apply an automatic emergency brake

Credit: Somerset Council

£70,000 has been spent on a complete redesign, and Dave Peake, the council’s highways service manager, said the issue was something that would "never ever have been considered" when the grid was put in "many years ago".

"It’s quite a steep hill and the cattle grid’s got to be reasonably level," he told the BBC.

"But the problem was this sudden change in gradient. Some of the car’s sensors were detecting this as a wall so automatically applied the brakes.

"So we’ve actually re-profiled it to take out the sudden change in gradient."

The issue in Somerset does not appear to be an isolated incident.

On one online forum, a Volkswagen Tiguan driver called Richard said that his front brake assist was activated in front of a cattle grid so sharply that “a closely following vehicle would have probably rammed” him.

Autonomous emergency braking (AEB) is designed to prevent, or mitigate the effects of, vehicle collisions. It is able to crudely predict when a crash is likely to occur, and then take steps to either avoid it or make it less serious. 

Generally, the system will sound an audible alarm, often with a visual warning as well, when a collision risk is detected. If no action is taken by the driver, or if the reaction is deemed insufficient by the on-board computer, the AEB system will apply the emergency brake.

The technology has been shown to have a remarkably positive effect in reducing crashes. 

At low speeds, it can altogether prevent a collision from taking place, and at higher speeds it can at least reduce the severity of the impact. It does this by constantly scanning the road ahead, and by amplifying any brake pedal pressure into an emergency brake signal if it thinks your reaction to a hazard has not been strong enough.

Prof Andrew Graves, automotive analyst at the University of Bath, said technology like "assist brake" had been around since the 1980s but "a lot of this technology is not clever enough at the moment".

"If you’re following something too closely or if there’s a barrier in front of you, suddenly the car will apply its brakes," he told the BBC.

"Modern cars are pretty good at doing this but they’re still not perfect and sometimes they send out a very confused message."

The grid has caused other problems for road users, and in 2019 police warned motorists not to treat it like a “long jump” after an uninsured motorist flew over the grid and into a tree.

Pictures showed a wrecked Volkswagen Golf with its front end crumpled being taken away on the back of a low loader.

PC Sam Donati said: “They were incredibly lucky that the tree this car piled into went between the engine and chassis leg. If it had hit either the driver or passenger, the outcome could have been very different.”

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