Our cat Fuzz died last year just before christmas. she was hit by a car. we didnt have time to get the vet round (it was just past midnight) and i had to sit stroking her for about 3 minutes untill she died. it sounds like your cat had the same injuries as fuzz, im so sorry, its awful to lose a pet that way. he could possibly have walked to the grass verge, or maybe someone carried him there, i dont know, but at least he isnt in any pain now. i know thats not very reasuring, but the pain of the loss does get easier in time.
oh and to answer your question, he would have been in shock, and so he wouldnt have felt anything. xxx