The cat should not have been on the road.
I feel for the op, a loss of any family pet is always a very distressing time. My thoughts are with you. Be grateful a nice neighbour and a professional vet did all they could so it did not suffer a lonely long painful death.
However people need to learn to take responsibility for animals more, if you live in a place where cars roam about, then let your cat go play with the traffic what do you expect.
Giving a cat a cosy bed, loving strokes, and succulent kitikat is not where the responsibility ends. Cats dont understand cars kill, you do.