The car, which is initially travelling at low velocity relative to the truck, isn't going to suddenly accelerate and smash into the front of the trailer at high relative velocity: for that to happen, it would need to pick up a lot of energy from surface traction in a very short distance. (Such a car would be able to accelerate zero to sixty in a trailer-length, which would be a sight to see.)
Instead, you get immediate wheel-spin when the drive wheels hit the ramp, and need to declutch and get into first gear before the engine stalls. Then you need to power it up the ramp and hit the brakes before you drive into the front end of the trailer at (relatively) low speed.
As I recall, the driver of the modified bus used in the original Italian Job was protected by a reinforced wall and a lot of padding, and was shifted forwards a couple of inches by the low-speed impact from behind.

Grant Hutchison