Here's my perspective as a bike rider with over 40 years experience (started aged 17, I'm now 62) and who hasn't driven a car since 1972.
In answer to those who think bikes are difficult to spot, I used to agree with you - until I bought an ex-police motorcycle, it had the 'Police' stickers and lights taken off but it retained the day-glo stripes etc.
Suddenly, instead of being invisible to car drivers, it felt like I had a personal follow-spot, beaming down from heaven, upon me. Cars would pull over to enable the overtake and tip their forelocks. Little old ladies smiled and children waved as I passed by.
Best of all, filtering between lanes (which is completely legal) ceased to be dangerous. Instead of being a one sided game of Russian roulette, it was like Moses parting the Red Sea - columns of traffic, miles ahead, would part like an overeager flasher's gabardine mac.
My experience suggested to me that it isn't that some people 'can't see' motorbikes, it's that some people simply don't care. It's why I now ride 'defensively' and assume that all car drivers are psychopaths. No offence but this means you!
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Helpful Hints
If it's got a loud, high pitched exhaust, it's probably a kid on a scooter not a proper biker. Most of the dangerous stuff you see is these kids. They behave like idiots because they're 17. If he doesn't die young, Mummy and Daddy will buy him a car in 6 months time and then he'll become an equally twattish car driver.
If it's got a very loud, low pitched, grunty exhaust, it's probably a Harley-Davidson. Mostly harmless but beyond help.
Most really fast bikes on the road these days are fairly quiet, due to regulation. Don't expect to hear them first, use your mirrors!
If it's very loud and clean but not a Harley, it's probably being ridden by an idiot. Expect to be overtaken on blind bends etc.
Filtering is legal up to 20mph. Get used to it. It's illegal above 20mph but you should still expect it.
Bikers aren't on the phone or twiddling with the radio. They sit higher and have a much better idea of what's happening around them than you do.
You can fall off a bike and break your leg at a standstill. A patch of gravel under a foot is enough put you down at 0mph. Similarly, a bike may not be able to stop or go where you think they should. A biker has to deal with road features that you don't notice - spilt fuel, odd cambers, slippery manhole covers, traffic markings - any of these things may force a bike to change direction unexpectedly (to you, but not them) on order to avoid an accident. Give bikes as much room to manoeuvre as you would a car.
If there's a bike coming up from behind, don't change direction to 'help' or avoid them. They don't expect it, so it's dangerous.
A biker may appear to be behaving like a nutcase/hooted/shouted/made a rude sign because he wants you to notice him/her. You may have failed to give them obvious eye contact.
If you're driving an automatic, don't drive up to junctions and brake hard at the last moment - it scares the shit out of bikers, especially if they've already suffered a SMIDSY.
Don't tailgate a biker, they'll get nasty. Having several tons of metal sitting on your arse is frightening enough in a car. Imagine what it's like for a biker...