Question 152632
{{{P=(1/4000000)^50000}}}
{{{P=(2.5x10^(-7))^50000}}}

a) That's a pretty small number, almost zero. Even just taking ten trips, the probability is {{{9.5x10^(-67)}}}. But it's not zero. There are assumptions made about an average trip. The assumption doesn't take into account length of trip, speed of the trip, seatbelts on or off, how safe your car is, daytime/nightime, traffic congestion, urban/rural, weather conditions, number of airbags, etc. A 12 hour trip on a high speed freeway in Nome, Alaska probably has a higher likelihood of fatality than a 3 minute trip through a school zone in St. Louis, Missouri.
b) The probability is so small, it shouldn't matter, but it does. 
On average, wearing a seatbelt increases the chances of surviving a crash, just like wearing a helmet while riding a bicycle or motorcycle.
That doesn't mean, strap on the seat belt and drive 100 mph into a brick wall though, because those odds and the laws of physics are against you.