Generally yes but I have had a car inches off my back wheel without me knowing. Dead straight narrow road, but due to wind noise in helmet straps, just didn't hear her.
Could be a reason for not wearing a helmet or get a mirror.
I doubt it was the straps, which I would have thought were too insubstantial to be the cause of wind noise. I would have thought it much more likely to be the movement of air/wind around the bulk of the helmet itself that was the cause. If so, I've noticed that some helmets have much less bulk close to the ears, e.g. the Kask Protone, and I suspect that that might significantly reduce wind noise making it difficult to hear vehicles approaching from behind.
robj7744 wrote:The radar is also good at picking up when there are two or three cars behind you. It means I know how many cars are coming past and am not surprised by a second car after one has passed.
When a vehicle overtakes me on a country lane, I do a shoulder check to see if there is another vehicle or vehicles behind that are also about to overtake.
Peronally I agree with the OP - it's a lot of money for something which is inferior to a mirror. A mirror provides a lot more useful information. Not only can you see the type of vehicle (car, van, lorry etc.), but also by observing it in the mirror is possible to make a judgement about how it is being driven (too fast, poorly positioned etc.), including whether or not it looks likely to leave enough of a gap when overtaking, and consequently whether it poses a greater threat than usual and might require an emergency manoeuvre. My general experience is that (without a helmet to cause wind noise) I can hear vehicles behind me from a long way off on country lanes, e.g. 200m, and can then keep an eye on them in the mirror as they get closer.