I had to responde re tehe wearing of bicycle helemts. It's a subject close to my heart, gvein that my mum fell off her bike going at about 25 miles an hour, bounced on her pelvis (causing multitple fractures) and then hitting the front of her head. Her bu=icycle helmet doesn't look that damaged, but when you examine it, you see how compressed the plysturene is on the one side rather than the other side. It (plus the initial impact on her pelvis, which must have absorbed some of the initial momentum) probably saved her life - yet she still suffered head injuries and for a long time we did not know if we wold ever get "Mum" back. Even now, 6 months later, she is still not back - probelms with finding words, a lack of confidence, difficuy in putting together concepts, agitation at the strangest of things.....
Helmets do make a difference.
Having said that, as an adult, I admit to still enjoying occasionally cysling wihout a helmet. However, I alwys make sure I am doing so in front of ds, in order to present him with a good example.
Car seats and seat belts are the same: we need to make sure that kids understadn how important apporpriate restraints are.
Mlojam - the law is that it is OK for the third child to go without - and it would be up to you to choose (although they still need to wear a seat belt. Personally, if it were an issue and you hade a child witth a front facing seat, I would put one in to the front seat (pushed back of course) and make the adult squeeze int to the middle seat.
I do think that a proper five set racing harness that one person desrbided as the only option in the back of a racing car should be the sort of thing that should be a specific exemtion. Even if a booster seat could be sequeezed in, I am sure it wouldn't be as safe!