That's why those of us who like to drive our cars fast spend a lot of money on safety equipment. However, all too often I see how well-intentioned modifications to the car's safety systems end up making the vehicle less safe for the driver and the passengers.
Before I get into all the ways to go wrong, let me start with an example of a correct race car safety system.
These days, a closed-wheel race car will most likely have a roll cage, a containment seat, and driver restraints such as the 5- or 6-point harness, a helmet, and a HANS device. There are more things to it, like window and center nets, and hand tethers, but I will only concentrate on the essentials here.
The roll cage does what its name promises, namely prevents the roof and the sides of the car from collapsing if the car rolls over. It also adds stiffness in case of collision with other vehicles. The cage is rigidly bolted or ideally welded to the car's frame.
A good racing seat will fit the driver snugly and support and protect her head, shoulders, ribcage, back, and legs. Ideally, the seat will be also firmly mounted to the car's frame.
Once these two things are in place, you strap in the driver with a racing harness, which is hooked up to the car's frame at the bottom and the roll cage at the shoulders.
Noticing a pattern there? Everything is somehow attached to the car's frame.
This means, that when the car stops abruptly (by say, hitting a tire barrier or a wall), everything that is firmly attached to the frame will stop as well. That includes our driver.
This is where the HANS device becomes important. HANS stands for Head And Neck Support, and what it does is keep the driver's head from continuing to move (and snapping off) when the rest of the body has come to an abrupt stop when the car hit that wall.
I find it ironic that it does not take a breakneck speed to break your neck. In a head-on impact against a wall, as little as 30 mph may kill you just like that.
To see what I mean, check out this video showing how much a HANS device restricts the forward motion of the head in a frontal impact at 50 kph (31 mph).
Having seen this, imagine for a moment what adding a racing harness to a stock car would do. Unless you pick the DOT-approved 4-point, which has a seam in one shoulder belt that is designed to expand, allowing for your body to twist and move forward in an impact, you have just made your car not safer, but more dangerous than it was with the factory 3-point belts.
Adding a harness to a street car is a popular modification in the import tuner scene, but this one is not as harmless as body kits, neon, and loud exhausts.
The 3-point restraints that you know from your daily driver are asymmetrical, and allow your body to lean forward and to twist, preventing spine and neck injury that would happen if you were rigidly strapped to your car (such as with a 6-point). An improper 4-point belt will also squish your internal organs, to make matters more entertaining for you.
Now while I am on a roll, let me say that same goes for putting roll bars in cars where your unprotected head may come in contact with the bar in a crash. This is why I always advise people against aftermarket bars unless they are either a) too short to ever reach it, even if the seat collapses in a rear-end collision, or b) will wear a helmet at all times while the car is in motion.