That is a sad, tragic accident. Unfortunately, it's all too common and not just with lawn tractors. I know of several instances where people drove into the back of large tractors, combines, hay equipment etc. Most of the time, the equipment had flashers and slow moving vehicle signs.Looks like it was a hilly road, a small lawn tractor would be very low profile and maybe hard to see coming up
on it over the peak of a hill.
Speed of a car approaching and slowness of a lawn tractor, now very much reaction time.
Really sad to hear about something like this.