Ever notice how time machines portrayed in movies have a ten-year shelf-life before becoming auspicious? Typically, if you have a time machine, you don’t want people to know you have a time machine, because Biff will steal it. So, you disguise it to look like something else.
In Back-to-the-Future, there was the Delorean, which looked weird even in the 80’s. Try driving that thing around town today: people are going to know you’re a time-traveller/pimp. Plus it didn’t even last the first movie without being tricked out. Now it looks exactly like nothing has looked ever. Assuming a large group of people don’t see the car emerge from a flash of light with twin streaks of fire running behind it’s tires; someone from the 1950’s is going to take one look at it and call the authorities. Any other century would assume it was a dragon, and there’s not always going to be bushes/billboards to hide it behind. How the hell hasn’t the Delorean not cause multiple vehicular homicide ever time it jumps? There’s no way of know when another car/pedestrian will be on the road, (if there is a road), when you time travel, and you’re going 88 miles an hour. It takes a long, clear stretch of road to use. If you’re going to the past, you can make an educated guess, but the future is completely unpredictable. My theory is that Doc killed dozens of people by accident, then went back in time to cover it up.
Then there’s the whole phone-booth routine: ala Bill & Ted/Dr.Who. No one uses phone booths anymore. You can’t even find one, and if you do: a hobo is likely taking a dump in it. In Dr.Who’s case, he can’t even take it outside of England without being noticed. To me, this is a larger intellectual hurdle than the whole space/time continuum thing. Time travel is impossible because Biff will find your time machine and use it to sex up your mom.
