

I know a guy who spent 40+ years in special effects. He now goes around giving talks/demonstrations about it all. He has a series of photos showing how a typical car is rigged to explode into a fireball.
Depending on what the director is looking for, steel horns are welded to the car frame inside each door, the hood, trunk, windshield, etc. The horns will direct the fireball out the car when ignited. Each horn holds an explosive similar to gasoline connected to a detonator.
After that, each window is wired with a squib, a small explosive smaller than a coin that will shatter the window about 1/10th of a second before the fireballs are ignited.
If necessary the hood, trunk, and/or doors are also wired with smaller explosives to pop them open immediately before the fireball as well.
All those smaller explosives are needed to get the doors/windows out of the way for the main fireball explosives. The fireball doesn’t have enough punch to push the doors open on its own, and it also provides significantly more control of the whole explosion. (You’re not guessing where the windshield might get blown to, etc).
No. THX-1138 would have been filmed decades before these tunnels existed. I think that was filmed in San Francisco subway tunnels that were still under construction in the early 70’s.
These are the Boston tunnels: https://en.wikipedia.org/wiki/Big_Dig