They were crossing they highway on foot. A seatbelt wouldn't have helped. Although, other options include:
-Looking both ways.
-Not crossing a highway.
-Finding a crosswalk.
-Not standing in the highway.
Of course I have come to the conclusion that accidents like this are bound to happen since the vast majority of the human race are complete fucking idiots and don't know any better than to cross a highway on foot at night.
Also, this is the worst ending to a game of slap-ass I have ever seen.