First of all, I don't advocate doing this. In a BO situation I think that low profile is the way to go. Hopefully you can get to your destination before most of the sheeples realize the crisis is real.
Having said that, I found myself in the early days of Iraqi Freedom as a civilian contractor with Blackwater, in Fallujah, with no Forward Operating Base to call home, in soft-skinned vehicles, with no comm equipment that could reach over a mile, for six months. At times it was like a bad dream.
We started with soft-skinned vehicles, when armored SUV's and trucks were in short supply, eventually my crew and I found a friendly U.S. Army Warrant Officer at Camp Ridgeway who saw our sandbagged F350 diesel on a visit and offered to help. He ran a maintenance company. He had welding skills and a lot of the Hardox and Armox armor that was used to make "hillbilly humvees." He had us come by the following two Sundays, when no one was around and he got into working on our truck with a lot of enthusiasm.
He welded plates into all the floorboards, shields up to window level on all four doors, a massive "push" bumper, and, best of all, a double-plated gun tub for the RPK, later PKM, later SAW machine guns mounted in the back. The extra weight hardly seemed to affect the trucks get up and go at all. In fact, I really liked that truck. We paid him with a bottle of Absolut that we'd picked up at the mostly abandoned at the time Baghdad International Airport (BIAP) on one of our runs.
I highly recommend a diesel if you are going to try to uparmor a vehicle of yours. The weight is considerable and only the 3/4 and 1 tons can handle it for extended periods of time.
We could accurately shoot the MG's from the tub towards the rear, while rolling, with little adjustment. Same if we stood up and fired over the front. When firing to the side, it was basically necessary to walk our rounds into the target with extended bursts.
As far as firing our M4's from the cab windows, our Aimpoints were our saving grace. With some practice we were able to get first round hits during drive by engagements rather consistently. The red dots were very valuable for the right handed shooters who had to sit on the right (passenger) side of the cab. You could hold the gun left handed and, no matter how awkward it felt, as long as the dot was on the target it hit it with very little reverse lead (effective up to 30 - 50 meters or so).
This worked in training and later proved valuable when SHTF.
Some additional, though admittedly unscientific, observations. We had plenty of 10" x 12" armor plates cut that we used for target practice. At 50 meters from a 14.5 M4 barrel 5.56 ball, blacktip, and even tracer penetrated the Hardox and Armox. The greentip put a significant dent in it, but did not penetrate. 7.62 x 39 from both AK-47's and RPK did not penetrate. 7.62 x 54 punched a very nice hole.
At 100 meters, greentip DID PENETRATE. So did ball and blacktip. Tracer only dented it. 7.62 x 39 from AK and RPK did not penetrate. 7.62 x 54 make a sweet hole.
Most laminated bullet resistant glass will be penetrated by the third rifle round hitting the same pane. It is only bullet proof for the shortest periods of time. Hopefully enough to drive out of the gunfire or, if your vehicle is down, to bail and run away. Do not think it best to stay inside a brokedick armored vehicle. It is not an Alamo and is a bullet magnet, or worse...a target for a well-aimed RPG shot.
The less stuff you have attached to your M4, the less chance it will get tangled in seatbelts, your gear, etc. Especially if you have to unass your vehicle in a hurry. Aimpoint good. Forward grips, lights, excess rail, complex slings, etc, not good. If you have the luxury of having several weapons, a relatively unladen "car gun" is not a bad idea. Shortbarrels are better than long for the same reasons. I like to think my Auto Ordnance M1 .30 Carbine with the folding stock serves this purpose well (stateside). You still have a semi-decent weapon for use should you have to dismount (ask the ghosts of the thousands of dead Chinese and North Koreans). While I simply love the MP5, and found it almost perfect for SWAT entry work, it isn't that great when you need to reach a little further. A shotgun slug will penetrate another car at close range, but 00 didn't prove very useful (maybe on crowds of zombies it would?).
Gunfire hitting your car frequently only sounds like someone tapping the side of it. You may not recognize that you are under fire unless you hear the actual report, see circular muzzle flashes, or see the little booger eaters themselves.
The best drivers should be designated as the wheel men. Good shooters are a dime a dozen, good tactical drivers are not so. Most people think they are good drivers. Ex-cops frequently are but anyone with good reflexes can learn. They also can't be hotheaded. Most people think high-speed is the cure, and speed is necessary at times, but dying in a wreck is dead just the same.
IED's can't always be spotted. Vigilance saved my ass more than once, but I've also been made aware of a roadside bomb by its detonation next to my car. It turns your world inside out. I was fortunate then and several times since. I watched the car in front of me (in '05 on a different gig near Route Irish) get blown off an overpass and onto the traffic below. They didn't make it. Armor can help with explosives (distance and shielding are your friends) but it only does so much.
Ignore flat tires until the car just won't go any further or you are sure you have reached a point of relative safety. A penetrated automatic transmission may not seize up for 800 yards or more, that may be enough to get you off the X. A destroyed radiator won't overheat the car immediately either, drive as far as you can. A car on fire (this happened, but not to me) went 40 kilometers before the smoke inside the car became too overwheming.
Travel only with people you love or trust. Early on over there, I met a lot of cowboys and drunks. They create more problems than they do solutions.
I write this to share MY observations and MY opinions. I know that others have experience in the Road Warrior worlds out there. Share what you learned. I lost a lot of friends (25 at last count) over there. People I actually knew and cared about. I have never met my fellow Tennessean James Yeager, but we shared a mutual friend , Jay Hunt, who is no longer with us. Three years over there was enough.
One last thing. Iraq was actually a fairly civilized country before SHTF. As Jack says, it is low probability that it will happen here, but it will be high impact. Just going to populated areas to get foods and water cost many people their lives, the markets were/are frequent targets for bombings. Get a good stash built up for yourself and stay away from the unwashed masses. Realize that there will be little to no electricity and no, it won't be back on "in a few days." Being there convinced me to be a prepper.
my 2 pennies,
Goatdog
