In my story a man who works in Military Intel gets early warning that a nuclear attack on his base is inbound. He abandons his post, grabs his wife and kid from on base housing, and books it out of there at 90 miles per hour with 20 minutes to spare. He is 30 miles away from ground zero when the strike happens. This is far enough away that he and his family are 10 miles outside of the air blast radius. The idea is that they continue driving as far as they can before running into clogged roads or running out of gas.

My question is, does the EMP stall their car? Does EMP affect cars? I know that EMP is not the magic anti-technology ray that Hollywood makes it out to be, but I don't know what the actual real world effects on a vehicle would be.

Depends on the car

It also depends on luck, sometimes an EMP will do permanent damage, sometimes it won't. The chances are you'll blow a bunch of fuses at the very least.

If you're running a diesel then it'll probably keep going. Not because the electronics are any more robust but because diesel doesn't actually need the glow plugs, it'll run on compression ignition. Depending on which fuses you blow, it may not start again if you stop the engine. If you fry the engine management system it'll run less efficiently, but it'll keep going. I once had a modern diesel running without fuel pumps, without glow plugs, without engine management, and only 3/4 cylinders, diesels keep going.

For petrol cars, the more modern and high spec the car, the more damage it's at risk of taking. If you're talking about a new BMW or Mercedes there's a reasonable chance it'll be completely knocked out from permanent damage to delicate electronics and drive by wire systems. Almost nothing you're holding is directly connected to the vehicle control it represents, it all goes through the computer first, even the steering.

If you're talking something from the 1970s with minimal electronics and all the refinement of a cart, it may even keep going with a brief cough as it cuts out during the pulse and then bump starts itself from the momentum.

Of course you're not the first to consider this issue.

The EMP Commission (appears legitimate) have written a report that includes considering the damage to vehicles from EMP which implies that there's no real issues.

However it should be noted that this report only covers vehicles up to 2002, current high spec vehicles have a far higher level of "drive by wire" computer control.

From page number 115 (pdf page 131)

We tested a sample of 37 cars in an EMP simulation laboratory, with automobile vintages ranging from 1986 through 2002. Automobiles of these vintages include extensive electronics and represent a significant fraction of automobiles on the road today. The testing was conducted by exposing running and nonrunning automobiles to sequentially increasing EMP field intensities. If anomalous response (either temporary or permanent) was observed, the testing of that particular automobile was stopped. If no anomalous response was observed, the testing was continued up to the field intensity limits of the simulation capability (approximately 50 kV/m).

Automobiles were subjected to EMP environments under both engine turned off and engine turned on conditions. No effects were subsequently observed in those automobiles that were not turned on during EMP exposure. The most serious effect observed on running automobiles was that the motors in three cars stopped at field strengths of approximately 30 kV/m or above. In an actual EMP exposure, these vehicles would glide to a stop and require the driver to restart them. Electronics in the dashboard of one automobile were damaged and required repair. Other effects were relatively minor. Twenty-five automobiles exhibited malfunctions that could be considered only a nuisance (e.g., blinking dashboard lights) and did not require driver intervention to correct. Eight of the 37 cars tested did not exhibit any anomalous response.

Based on these test results, we expect few automobile effects at EMP field levels below 25 kV/m. Approximately 10 percent or more of the automobiles exposed to higher field levels may experience serious EMP effects, including engine stall, that require driver intervention to correct. We further expect that at least two out of three automobiles on the road will manifest some nuisance response at these higher field levels. The serious malfunctions could trigger car crashes on U.S. highways; the nuisance malfunctions could exacerbate this condition. The ultimate result of automobile EMP exposure could be triggered crashes that damage many more vehicles than are damaged by the EMP, the consequent loss of life, and multiple injuries.

One other consideration is the car in the line of sight of the nuclear blast at detonation or not. Estimating a Nuclear Bomb's destructive capacity is a very imprecise science. For example, the Nagasaki device (Fat Man) was theoretically more destructive than the Hiroshima device (Little Boy). In reality, the reverse was observed. This was because Little Boy was detonated over what was an open field with little to stop the destructive forces of the bomb. Fat Man detonated below the mountain tops of a valley, which contained much of the destructive energy of the blast to the valley's sparing the bulk of the city, which could still witness the destruction.

The destructive portion of the bomb's blast are light/heat radiation and the over-pressure. In order, you will first see the light, then feel the heat, and then see the overpressure. The former two are near instantaneous. if you are in line of sight, I personally call this the "Simba's Kingdom Fire" because everything that the light touches will burst into flames. The closest known survivor to Hiroshima Ground Zero was in a bank vault at the time of the raid and was not exposed to the light/heat as the rest of the bank became engulfed in an inferno and then blown away. It's distance to GZ was measured in meters. The overpressure will be felt at progressively larger time gaps from ground zero. An air burst is more destructive than a ground burst because there is little to stop this, though the destruction of most of Nagasaki was prevented by obustructions that contained the blast, the affected industrial area was absolutely destroyed. Meanwhile, damage from the light at radiation at Hiroshima could be traced all the way to a grove of trees on a hill overlooking the Jinkaku-ji Temple in Kyoto... a distance of 361.2 km (225 miles) from ground zero.

This is true of the EMP as it is line of sight only... it was believed that a full on Soviet nuclear war would start with a very high altitude burst over the continental US... high enough that no target is damaged... but almost every electronic device is put out of commission on the North American Continent. The car may however still survive if something was obstructing line of sight from the blast. Being under a tunnel might give you the chance to get out of the way... the EMP in a full on attack on the US will normally proceed the attack's more destructive capability. A closer to earth air burst will likely be safe if he is safe from the light/fire destruction.

Powertrain: Dumb luck will be the ruling factor

It's all going to depend on the luck of the draw and how the hit occurs and how EMP waves bend around local terrain and the metalwork of the particular car.

First, remember the powertrain is already substantially inside a Faraday cage. The hood is metal, the unibody is metal and so is the engine - this largely wraps the engine package.

Nonsense to dismiss: that some cars are "mechanical" and will Just Keep Working. Nosirree. Every modern car is FADEC. Even the diesels, remember when VW cheated the Smog tests by detecting when the car was on a test stand and detuning the engine during those times? The only way to do that is with FADEC. All the other builders who were not cheating also required FADEC. So all modern cars are computer controlled.

I.E. the computer decides when (i.e. which microseconds) the fuel injector fires, and it does this in real-time on the fly. If the computer gets borked, the car doesn't stay at the last throttle position, it simply forgets to inject fuel starting right now, so the engine dies instantly.

The days of there being a basically mechanical carb/FI system with computers "only doing trim optimization" started in the 1970s with carburetors, and ended in the early 1990s -- killed off dead in 1996 when OBDII became law. OBDII requires FADEC.

But even in 1991 you'd be hard pressed to find a non-FADEC engine, probably in a light truck. Why? It's a darn good technology that makes engines run better, much longer, and more powerful too, by enabling other performance tech.

Older automobiles are practically extinct on the road

The current mix of automobiles found on the highway contain essentially zero pre-FADEC era cars. You might find older trucks without FADEC, the bigger the more likely, but only up to a certain size.

California has achieved a practical peace with classic car lovers, based on the following fact: There aren't enough of them left to make a difference. No, I mean there are loads. But they're not daily drivers. They run so rarely that they don't have a significant impact. Simply put, they're not worth regulating.

As an author, that means if you prioritize a search for an EMP-proof car, look for well-cared-for but rarely-driven collector cars. You won't find one in an office parking lot, though. It'll be in an enthusiast's garage. Or out back up on blocks, but that one won't run.

Creature comforts: All bets are off

Powertrain computers on modern cars are already extremely hardened - not against EMP but just against failing generally. Some fool shorting an oxygen sensor to 12V, a wiring harness meltdown in transmission control wiring, whatever. That's powertrain.

Now modern cars have a whole second *tranche of computers in the carbody, and those don't run the powertrain, they run the human/creature comforts. For instance the left door has 1 power window motor and Four power window switches. 14 wires? Not anymore, now it's 2 power/ground wires, and two thin little signal wires, and embedded computers in every door that chat over the signal wires.

And in recent years, as you well know, this "body-side" stuff has exploded to be the reason you buy a car - built in Nav, Siri, Alexa, smartphone integration, your music, Sirius Radio, all the smart-car stuff potentially including the autopilots - some cars more than 50% of the cost of the car is in this consumer-appeal electronic cruft.

This stuff is all body-side, and there are places where the body is not steel, but the dash and door panel interiors are not steel at all. EMP will ignore all that and reflect in unpredictable ways off the parts of the car which are metal, exposing the body-side stuff to a lot of potential damage.

So there's a pretty good chance that your engine will work but your speedo will not. Sirius works but ONStar does not, turn signals fail, and you can't roll down your windows. And the A/C won't turn on because the sophisticated comfort control took a hit, that doesn't doom you, but the fan won't run either, and that dooms you.

Ever drive a car with no fan and no windows in the summer? You cook. It's the same exact physics as dogs and babies dying from being abandoned in the car, except you happen to be moving.

However if the blower works, and a smart person could "jumper it so it's hard on" depsite blown electronics, that makes the car interior livable - though if it's a sunny day over 60F, you will need quite a bit of hydration, solar load beats you up bad. Actually for best freeway fuel economy you want windows up, A/C off, blower on max. "Windows down" is bad due to aero-drag, and A/C on takes a lot of fuel.