Cars are capable of more than 70mph, many drivers, depite their assertion that they are "Formula 1" material, are not . With a speed limit, drivers are able to make good judgement calls on what to do vis-a-vis other people on the road. How many deaths a year from driving? Quite a few Certainly, a massive amount more than guns are responsible for. I reckon the Ambulance services and Police view speeding as fairly serious, as they are the ones who have to scrape victims of it up afterwards.
Its just unfortunate that the law has not updated to make killing via a vehicle murder/manslaughter as appropriate.