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this.
I got badly concussed as a result of being hit from behind by a speeding motorist, the helmet was split in half and I had a concussion for about 6 weeks with eyes that looked like a slow loris's but the wonderfully inattentive shits at Harlow General where I'd been carted to on a spinal board didn't bother to tell me that, it was the horrified GP I saw two weeks later who was amazed to find me able to wander in to the surgery complaining of feeling sick and lightheaded under my own steam.
The helmet question wasn't and will never be part of the equation for me-it was shitty driving, poor medical care and then subsequently a total lack of action on behalf of the police that characterised the cause and effect of the whole situation and the quality of my recovery and I could just have easily broken my spine or been run over by the following car that skidded to a halt about a foot from my head.
OP- it took me almost a year to return to normal, things like balance, emotional state and vision were really badly impacted so I'd say a six month hiatus on driving is pretty reasonable although I did cycle from about 2 months after just because I couldn't afford to get around any other way.
Please let's not turn this in to a helmet debate.