Home made telescopic rig, set on a timer for a 1/2 second exposure. Camera set on the rig, with an off camera delayed flash at a its lowest power, dad in the driver seat, pushed the car so not to produce any vibration from the engine, maybe 1mph thats about it really! Its paramount that the car is not vibrating as the longer the rig the more vibration you get from the suction cups on the car. Trial and error this was the best shot. Then photoshoping the rig out of the shot and some post processing.
Don't want to give too much away about how I set up the rig etc because I do it for a living so don't want everyone knowing but if you want to know a bit more about car rigs give it a google.