G-ALAN...
Here's a way to check it out.
On a nice dry day, accelerate your car to 70mph. Next to a marker post hit the brakes - HARD. Try to avoid locking the wheels - or preferably, use a car with anti-lock brakes. Now measure how long (in yards) it took to stop.
Now move your post to this many yards from a brick wall. Wait for a nice wet day (plenty of those at the moment). Accelerate your car towards the brick wall and hold a stready 70mph. When you get to the post, brake as hard as you can - again, try to avoid locking the wheels.
Let us know how you get on!
NoD