theshrew - I'd love to know your evidence for saying that a crate in the boot is not safe.... as that is what I have always been told is the safest way for a dog to travel (as long as the crate itself is secured) and is certainly how "professional" dogs are transported....
With 2 children under 2 on the backseat, there is no option for Honey to use her car harness anymore, so we have secured her spare crate into the boot. 
I'm guessing the evidence is any of the cars that have their rear end crushed in a shunt. As somebody else said; modern cars are designed to crumple to absorb the impact energy.
No car is 100% safe - for people or dogs - but statistically you are more likely to be rear ended than roll your car over.
With regards to this, and many, many other areas of life, I don't do something just because lots of other people do. I make an assessment of the facts and decide what I feel is best for me and mine. If I had a Land Rover I would probably use a crate. If I had a Smart I probably wouldn't drive it over 30mph.
Seat harnesses aren't ideal either and have drawbacks in other situations - such as a rolling car - but I'm working on a new option to the standard harness. Hopefully I'll have a product on the market before the end of the year. It will still have situations in which it won't be ideal, but then that's down to each of us to assess the risks based on our own vehicle and driving experience.
Just out of interest: has anybody ever been in a rearranging crash in a hatchback and ended up with an unopenable tailgate? Its happened to me, but fortunately before I had a dog.