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Blind spot assist – how does it work?

Blind spot assistance warns drivers when a vehicle is to their side and can’t be seen in the wing mirror. Here’s what it does and how.

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Blind spot assistance is a recent technology to warn drivers when a vehicle is to their side and can’t be seen in the wing mirror, or fast approaching. Here’s what it does and how it works.

The term ‘blind spot’ describes the point in a wing mirror at which another moving vehicle momentarily can’t been seen. Blind spots can be large enough in size to easily block another car, motorbike, cyclist or pedestrian from your view. 

It’s drilled into us when we learn to drive to check the blind spot and on the emergency stop, you can be failed for not checking both the nearside and offside blind spot. But over time, when changing lanes, some of us will have jumped to find a vehicle to our side that we hadn’t seen, or much worse, had an accident. 

New laws require a minimum level of assistance technology

EU law requires lorries to be fitted with specific blind spot mirrors, but this doesn’t apply to cars. Stick-on blind spot mirrors are easy to find at your local automotive supply shop but results tend to vary.

However, since July 2022 all new designs of car introduced into the UK market follow the EU directive on advanced driver assistance systems. This means they must have items such as autonomous emergency braking (AEB) and lane-keeping warning/assistance.

This also includes systems variously called blind spot detection, blind spot assist, blind spot monitoring or side assist, depending on the manufacturer. Once only fitted to premium brands, many mainstream cars have added blind spot assist systems in recent years. Some companies now also offer to fit blind spot and lane assistance for older cars.

Typically, a blind spot assist system checks the area to the rear of the vehicle on both sides by signalling the presence of vehicles approaching from the rear in an adjacent lane, usually at 19mph or faster and notifies the driver by turning on a warning indicator light in the mirror and in the display ahead of the driver. If you move the indicator lever to signal a turn in the direction and the blind spot warning indicator light is showing while an approaching vehicle is detected, a warning sound is activated. 

How a blind spot assist system works

They work by using radar, typically in the rear corners, sometimes, all four corners of the vehicle hidden behind the bumpers. While front-facing radars used for automated emergency braking and adaptive cruise control may look up to 200 metres ahead, blind spot radars are shorter range, normally 50 to 100 metres. 

Thatcham Research, which is Euro NCAP’s UK member organisation and funded by the insurance industry has devised an Assisted Driving Grading, in addition to the Euro NCAP safety ratings putting today’s Assisted Driving systems through their paces to independently assess their strengths and weaknesses. 

Tom Leggett is Thatcham’s vehicle technology specialist. A former crash test engineer, he was Thatcham’s lead research engineer for automated driving and now follows the development of safety technology, looking to see what new assistance/safety technology is on the horizon and what that means, especially in the move to electrification.

Blind spot assist isn’t related to parking sensors, because they work best at very low speeds and use ultrasonics. Radars detect faster moving objects. “Particularly things like motorcyclists,” says Tom.

“They can be quite tricky to detect because they are quite small when compared to a big boxy vehicle. A corner radar can do a really good job of detecting where a motorcyclist is.” Because radars can be hidden behind body panels they are also shielded from dirt and parking knocks, but they can be affected by severe weather and will flash a warning if so.

The position of the radar sensors is carefully mapped to the dimensions of the vehicle, measuring the angle at which to point from the rear part of the front doors. These shorter-range radars have a very wide field of view rather than a narrow view down the road. This allows them to detect what objects are coming and where they’re moving. 

Some systems can look across two lanes or more in case a vehicle moves into the blind spot from across a motorway. However, Volkswagen’s Side Assist ignores vehicles which are more than one lane across, so users are not distracted with frequent warnings, it says.

It only works from speeds above 40mph and is primarily designed for motorway driving. It’s also worth noting that blind spot assist systems switch off if you are towing, and as yet can’t account for the length of a car plus trailer/caravan.

While reversing cameras and parking sensors have become essential on some cars styled with thick rear pillars so you can’t see where the corners are, Tom says this has not driven the fitment of blind spot assistance systems. “You wouldn’t really look over your shoulder anyway. I don’t think it’s necessarily driven by vehicle design, more driven by providing the driver with more information when they are making a decision to do something.”

Blind spot assist began through safety concerns but is essential – like other assistance systems – for future automated driving technology (self-driving cars) like automatic lane changing.

“Without knowing what’s in your blind spot, the car is never going to be able to change lane for you, or help you change lane,” says Tom Leggett.

“More advanced systems are able to actively prevent the driver from colliding with a vehicle hidden in their blind spot by steering the vehicle back into the lane, similar to lane keep assist.”

Lastly, like all assistance systems, blind spot assist should not be relied on entirely. It’s your responsibility to be aware of your surroundings at all times. The majority of systems don’t work when you are stationary (although some Mercedes systems will warn you if you open the door at the wrong time) so if you’re moving off from parked, check your mirrors and look over your right or left shoulder. 

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Russell Hayes
Russell Hayeshttps://amzn.to/3dga7y8
Russell Hayes’ early career was 14 years of motoring journalism in print, television and online. He worked for What Car? and Complete Car magazines, the BBC's original Top Gear programme and Channel 4's Driven. Since 2007 he has written motoring history books on subjects including Lotus, TVR, the Earls Court Motor Show, the Volkswagen Golf, Volkswagen Beetle and Bus and the original Aston Martin V8. Now a full-time author, two more books are in the pipeline for 2023 and 2024.