So you might have thought putting a nice shiny 4-point harness (or 5-point, or 6-point) in your car was the right thing to do. Maybe you just bought some sport seats and the old buckles are still attached to the factory seat. Maybe you realize that being firmly fastened into the car allows you to keep your hands light on the wheel, and promotes good driving technique. Whatever the reason, there are some potential drawbacks you should know about.

1. Installation must be perfect. Mounting points must be very secure in the direction of tension (impact forces over a ton), and belt angles are critical. Typically the shoulder straps should be close to horizontal and the lap belt straps should be at a 45 degree angle downward (often a bit more). Expect to lose the use of the seat behind the driver.
2. Any harness must be very tight on the hips. It should be what most people would consider uncomfortably tight, or it is very dangerous to wear. This makes driving with a full bladder painful, but the lap belt pulling even an inch or two above your hips in an impact will literally crush your internal organs, which is a very bad way to die.
3. Consider the time it takes to snug a harness down properly. if you think you’d be inclined to get lazy and not wear it occasionally, or not quite tighten it all the way, don’t bother installing one. If you skip wearing a seat belt sometimes, this is definately not for you.
4. Four-point harnesses can be unsafe by design. A “crotch strap” in a 5/6-point harness actually exists to oppose the upward forces on the shoulder straps in an impact because the lap belt is at an angular disadvantage in doing so.
5. 3″ webbing (and probably 2″ as well) can cause severe neck lacerations in an impact. Racing suits have fabric reinforcement to resist this, but wearing a t-shirt and a harness could be a recipe for a severed carotid artery (also not a great way to die). Seat belt pads aren’t just for ricers in this case, and might reduce impact to the clavicle too.

I installed a harness in my beater because I don’t trust the stupid early-nineties “automatic seat belts” to not kill me. I weighed the risks and the inconvenience of having a harness in a commuter car, and you should do the same — after all, it’s your life.