Doesn't Norway have huge taxes on automobiles which chill ownership? If so, then population is not relevant. If only 10% of people can afford to own cars, then the number of crashes and fatalities will certainly be low.
The US standard of living is far higher. Whether that leads to more fatalities or not is anybody's guess. I would prefer the idea that we don't bother teaching people to drive very well or that people don't value developing their own good driving skills.