Earlier this week, I passed what looked like a pretty serious car fire on the shoulder of I-95, just north of Baltimore. (people were out/ away from the car, and on their cellphone, but fire/ police had not yet shown). The car in question looked like it had been a fairly late model Audi SUV/ crossover.
Since it looked relatively new, my question: what would be likely a) causes and b) warning signs of an impending car fire on a (presumably well maintained) new/ new-ish car?
The smell of fuel and/or white smoke (gas on a hot surface) is a pretty good indicator that a āthermal incidentā is about to happen. Stop the car and get out and away from the car, quickly!
The causes are many. An assembly mistake with quick-connect fuel lines. A nick in the plastic fuel pipes that breaks open. An o-ring failure.
Engines are very tightly packaged (especially Audis) and leave little room for the routing of fuel lines. Fuel injected modern cars can pressurize fuel to 75 psi. Modern direct injection autos pressurize the fuel to even higher levels. Even a small leak can cause a quick and catastrophic fire.
It isnāt all that common and the car is unlikely to explode like you see in the movies but you should still back away a safe distance.
Many thanks!
+1 to @Mustangman. Car fires start for a lot of reasons, including cigarettes dropping on the seat. New cars and new furniture in homes are made with polyurethane, poly blah blah and other plastic based materials. Once these start to burn they take off. Pay attention to any burning smell, especially electrical burning smell. Phone charger and GPS charge cords can get nicked. Keep cigarettes in the ash tray - if your car even has one these days. Next time you are at a big box store pick up an ABC fire extinguisher, usually less than $30 and keep it in the trunk. When the fire just starts and is small you might be able to knock it down. Gas tanks rarely explode. The lift supports for rear gates on minivans etc will explode and go flying when they get heated long enough, I have seen them fly 20 feet. Have not seen it happen but have heard about bumpers flying off when the energy absorbing āshock absorberā heats up and explodes. Once the car starts burning beyond what a small fire extinguisher can handle get away from the car, stay out of the traffic lane, try to move to the front of the car 50 feet or more away. Do not stay behind it, too many idiots will be mesmerized by the fire and plow right into the back of the car.
freebasing while driving.
Acrid smoke.
Seriously, thereās no way to guess what the cause might have been.
And thereās no way to predict a car fire until the acrid smoke starts.
As usual I agree 100%. I have thankfully only had one minor car fire. In 1970 I was driving my 1963 Ford Galaxie 406 cu in 3X2V 405 hp Super High Performance in a Western Oregon business district. Of course it was raining. I noticed rain drops on the hood center were suddenly drying. I pulled to the curb and shut off the engine. When I opened the hood and saw the exposed paper air cleaner element was on fire! There was no prior backfire. I closed the hood and ran into a nearby dry cleaner I used their phone to call the recently activated 911 emergency number reporting the vehicle fire. When I went back outside a Bell telephone van driver had noticed the smoke and stopped. The driver was correctly discharging his CO2 fire extinguisher under the front bumper. He then opened the hood and seeing the air cleaner smoldering gave it a couple more shots. I thanked him and he left. Within 30 seconds the fire truck arrived and I told them what had happened for their report. One fireman quipped that the telephone company was stealing their business and commended me for noticing the drying rain drops.
Yikes CRAP I am so ignorant,
The only car fire Iāve ever had was in a buddyās Studebaker in the '60s. We were trying to āpick upā a couple of girls and the engine suddenly backfired and the carb caught fire. The girls walked away. We put the fire out and drove away.
The '60s were fun! 
The engine wiring harness on my cousinās Ford Maverick caught on fire
She was living in a trailer park at the time
Talk about adding insult to injury 
Later on, she inherited a lot of money, bought a fancy house for cash, and basically disowned all of her relatives. Iām not even sure why. Everybody else was in much better financial shape and didnāt need any of her money, anyways. I suppose she was just paranoid. Or maybe she just thought she was better than the rest of the family.
Actually, Iāve got a few relatives whoāll treat family even worse than they would their worst enemy
Who needs enemies, when youāve got relatives? 
I went with a friend and his father to pick up a new 57 Country Squire. The trio home started badly and then got worse.
When I got in I stepped on and crushed the aluminum sill plate. As his father was driving, the car backfired amd we could smell something burning. As he pulled over a large brown spot appeared on the white hood. He popped the hood and we tried to get out of the car and none of the doors would unlock. By the time we got out the window it was apparent that he would be needing a different new car. It was only a mile from his house to the dealer. We left the keys in the car and called the dealer from his house.
I never rode in one of his cars again, he was offended when I couldnāt stop laughing.
I would venture to say that most car fires are started by fuel leaks or due to an electrical problem related to a poor connection which causes arcing or a high amperage draw which is overheating a component such as an ignition switch.
Ford recalled countless vehicles over the cruise control switch on the master cylinder. This was causing vehicles to catch on fire even while parked. In OK City 2 homes were badly damaged due to this problem. One was an F150 and the other a Ranger.
The Ranger took out the garage and the F150 took out the garage along with the kitchen.
Both incidents happened late at night while the owners were in bed. Luckily no one was hurt.
Iāve gotten a couple of Recall letters on my Lincolns for this very problem. The letters advise me to park the car away from structures, trees, or other flammable things until the Recall is performed.
I thought at one time my old Mercury Sable was going to catch on fire. The ignition switch was overheated and melting.
The cause of that was poor electrical engineering due to running the cabin blower motor current through the switch instead of a relay. After replacing the switch I wired a relay in to cure that engineering faux pas.
We had a 57 Fairlane and I recall they had problems with the choke on them sticking. It would cause stalling but not sure how it would cause a fire but for that short a distance it must have been gas spewing out.
Just a warning, I got flagged for simply saying that a post was some years old. So the computer made my message disappear pending review to see if I should be suspended or not. Said multiple flags on that message for being off topic. Canāt see who flagged it but it would be nice if someone has a problem they just say so like the same and others. At any rate there might be some folks around just trying to cause trouble.
Would that be a Richard Pryor reference, perhaps?? 
LOL, I didnāt think of that! Good one! 
Perhaps there are fewer car fires without carbs and with less smoking. My brother in law had a one or two year old first generation Tornado. He thought he had thrown his cigarette out, but it landed in the back seat. By the time he noticed it was too late.
I once saw a car backfire at a light and form the brown circle on the hood. The driver jumped out and attempted to smother the small flames with his new looking jacket. The fire burnt a nice round hole in the back of the jacket. He failed to see the humor.
Iāve also seen car fires caused by poorly-installed high power audio systems.
Car fires can also be started by overheated catalytic converters.
Or even hot cat converters parked on tall dry grass.
This was a serious problem years ago before manufacturers added heat shields.
+1
About 20 years ago, while driving on US Route 1, I was sitting next to another car at a traffic light.
In my peripheral vision, I saw what looked like a fire.
It turned out to be a couple of white-trash types who were ignitingā¦somethingā¦in a glass pipe, with a propane torch.
As soon as I figured out what I was looking at, I floored my gas pedal, and put as much distance as possible between myself and those druggies. 
However, I have to point out to the OP that a dropped cigarette buttāor even a dropped MJ jointāis also a possible cause of a vehicular fire. If one of those items is dropped and is not quickly retrieved/extinguished, it is very possible for the situation to result in a conflagration that encompasses the entire vehicle.
In any event, when a driver or passenger is addicted to nicotine, or marijuana, or to something of a much āharderā nature that is āsmokedā, that act of smoking in the car has a real potential to result in a vehicular fire.