This doesn't weaken the argument. We already know that there was no speed limit before this one was set. Knowing that the cars a decade ago were able to exceed 90kph doesn't tell us anything useful. Presumably, the cars built today are also mechanically capable of going above 90kph. None of this information tells us whether the speed limit reduced the accident rate.

This doesn't weaken the argument. We're not interested in accidents occurring on other roads. We just want to know whether the highway speed limit is responsible for reducing the accident rate on the highway.

This doesn't weaken. Even if most people go above the speed limit, they may still be going slower than they did with no speed limit at all, in which case the speed limit might have helped improve highway safety.

This weakens the argument. It tells us that at the same time at the speed limit came in, there was another factor--changes in car design--that improved driving safety. So we can’t be sure it was really the speed limit that caused the 15 percent reduction in the highway accident rate.

Irrelevant. Seatbelts and airbags affect what happens after an accident occurs. We want to know about what affected the rate of accidents occurring in the first place.