Thanks for your advice.
Liz, I'm too scared to look at the youtube clips.
As I said, taxis rarely reach 40. They just crawl along. It's very frustrating being a passenger. I think the capsule on the floor may be the safest. I don't know why I didn't think of that, I was only thinking of the capsule sliding off the seat and bouncing around everywhere.
As for booking a particular taxi.... it's a nice idea but things are really different here. The usual practice is to walk to the street, call a taxi by phone while trying to flag one down. You jump in the first one that stops. Likewise, a driver that accepts a booking will stop for the first person he sees.
Kate07, I hope you enjoy your holiday in VN. The traffic in Cambodia is a little different. I think they drive slightly faster but the traffic overall is much saner. PM me if you want any travel tips!