While in Iraq, my platoon had some nerve wracking experiences with suicide car bombers and insurgents trying to ram vehicles and gates. If you let a vehicle, even a sedan, get up to a decent speed-30mph or more within a block, then you are probably not going to stop it short of it's target without a hell of a barrier. One of the gate guards in one incident fired five rounds from his Mossberg 500 (18 inch barrel) loaded with #4 buck at 30 yards (measured from when he turned toward the gate at the time of the incident) and it didn't break the windshield. The windshield was broken by the insurgents head when he rammed the jersey barrier with his sedan and the bomb failed to detonate due to he remote tearing loose in the impact. Another incident happened when a suicide car bomber came barreling down a side street intent on impacting the humvee behind me. My only indication on what was going on was when the 240B opened up on it prior to it exploding 20 feet from the vehicle. I saw the sedan as impacts of 7.62x51 and 5.56 peeled back metal on and in front of the hood and spiderwebbed the windshield with the driver and passenger jerking when it erupted in and orange flash, smoke and dust engulfing the humvee. Luckily it was the newer 1114 up armored humvee and the crew survived with a major concussion and what we would later find out as TBI (traumatic brain injury) and no purple heart to show for it. We found out you need large solid barriers and/or large caliber weapons to stop a determined attacker in a vehicle and even then they might still get close enough to do damage. As some have indicated above, slowing down vehicles is a great way to mitigate it's effects. Sand traps, ditches, sharp S turns, spike strips, multiple cable fencing, and iron fencing. I'm not a big fan of concrete or hesco barriers though, as they can provide cover and concealment to attackers.