Speed calculation

[modeller]

It is so many years since I did any physics calculations that I would like your help.
My neighbour's house has a fairly steep drive 1:5 which is 20m long on a firm surface. Her estate car was standing at the top of the drive when she said her child got into the car and released the hand brake . The result was the car ran backwards down the drive across a busy lane , went over a 30 cm hump, ploughed through an overgrown , sapling covered area , and ended up 6 metres inside the area. It took the AA couple of hours to get it out and back in the drive.
No major damage was done, apart from the flattened saplings, and no one was hurt .
She was able to drive away so I assume the car is OK.
However to have jumped over the hump and crashed through all that wooded area must have required considerable force . Therefore I would like a rough estimate of the speed of the car at time of impact. I say estimate as I know tyre/road friction and car weight etc. would be factors.

[ChuckFickens]

Not had to do this for a long while!!

By my reckoning the car would have an acceleration of 1.87M/s/s and would have travelled the 20m in about 4.6seconds with a final speed of 19.3Mph

This doesn't take into account the bump and assumes a COF of 0 (the weight of the car doesn't make any difference anyhow)

[jomifl]

A similar thing happened to a friend of mine, It cost more to repair the house it hit than to repair the car, the child was unharmed. My guess was 20 mph.

[modeller]

Thank you very much it's amazing the increase over such a shot distance. I've tested it out in my car now and I can confirm it was fast before I had to slam the brakes on.