Captain Williams drops some bombs on some buildings near the highway to try and stop Hart and we see them all explode and produce black smoke. When Hart approaches the buildings a little later, they are exploding again.
The protagonist refuels his Porsche by pumping out the remaining fuel from storage tanks that have apparently not been refilled for 20 years. Gasoline tends to lose its combustible tendencies in as little as six months - depending on environmental conditions - and after two decades would definitely not be usable in a vehicle engine.
Captain Williams crashes his jet into the weapons bunker with enough force to completely destroy both it and his jet. Yet the flimsy glass canopy is found later on the ground in almost pristine condition.
While Capt. J.G. Williams flies his kite on the rooftop traffic noise is heard quite clearly in a world with no cars.
In the amount of time it would have taken Captain J.G. Williams [Burgess Meredith] to fix up and paint the jet that he flies, Franklyn Hart [Lee Majors] and Ring [Chris Makepeace] would have long since arrived in California.