I remember back the the 1980s when U.S. lawmakers ordered that new cars must have a third brake light. Do other countries require the third light? If so, which ones? If they don’t require it, it is installed anyway?
from here:
Canada does. I think it came into effect in 1986.
India does not require, by law, a break light in the centre at the rear of a vehicle.
Most factory built cars do not come with this option either. It’s only the sports models of some cars that have a centre brake light either in the centre of the spoiler or inside and centre of the rear windshield. And it’s an option that can always be fitted at anytime if someone feels the need for one. Some people think that dancing break lights (ala Night Rider) are cool, so they install it for the looks. Some feel centre break lights are an added safety measure, especially on the highways, so they install it for this reason.
Hella used to sell high mounted brake light kits for some VW and Audi vehicles before the third brake light became mandatory. I had them on my Rabbit which seems awfully nerdy when I look back on it now.
I think they are now compulsory here. Interestingly, my understanding was that high centre brake lights were found to be attention grabbing (and therefore accident preventing) when they first came out. But I read somewhere that after it reached the point where they were commonplace, the benefits stopped. In other words, they were only useful when they were novel, and now they are a waste of time.
This page at the National Highway Traffic Safety Administration site talks at some length about the effectiveness of “center high mounted stop lamps” (CHMSL), as they’re formally known.
According to that site, the primary motivation for mandating them (new cars in 1986, extended to light trucks in 1994), was three tests of the CHMSL’s in taxi and company fleets done in 1983. Those tests showed a 48-54 percent reduction in “relevant” rear-impact crashes - whatever that term means.
A study based on 1986 data suggested that cars with CHMSL’s were 15% less likely to be struck in the rear than ones without. A similar study using 1987 data showed an 11.3% value.
The Insurance Institute for Highway Safety did a study based on 1996 data which showed the value to have reduced to 5%.
The NHTSA went back and thoroughly analyzed their data themselves, and concluded that the Rear Impact Reduction dropped from its 1986 level to a steady-state value of about 4.3% by 1989.
They then bombard one with tables and statistics, closing with:
Feel free to critique their numbers. I’m just pointing 'em out. 
Correct. New vehicles sold in Australia must comply with Australian Design Rules. One of these rules mandates a “centre high-mounted stop lamp”.
Well, the centre brake light on my 1999 Cherokee didn’t stop a driver from ramming me from behind when I was sitting stopped in traffic (with my brakes on) a couple of years ago. (In Southern California it’s apparently more important to find a good station on your car stereo than it is to pay attention to such details as operating the vehicle, noting traffic conditions, etc.)
Maybe we need to add more brake lights. 
As I remember it, the reason for the centre brake light is that it clearly indicates at night that you are braking (when the taillights would normally be lit anyway, and there might be some hesitation by the following driver deciding if those lights mean the vehicle is braking or just that the owner uses very bright blubs). The centre brake light, lit only when braking, is suppose to prevent this hesitation.
And even though I’m from NY, I prefer using the ‘centre’ spelling 
Excellent. Our agents are already having an effect. Soon you’ll be reckoning your distances in kilometres, your milk in litres, and adding ‘eh’ to the end of your sentences… 
dopers,
i have taken it one step further and installed a fourth brake light that rides in the trailer hitch reciever and plugs into the trailer light pigtail on my truck, i have seen the inside of an ambulance and i do not care to see anymore.
unclviny
Hmmm… maybe Johnny’s got the right idea: whenever the effectiveness of a new brake light drops off, we just mandate that a new one be added. When the entire rear end of a vehicle is required by legislation to be covered with lights, noone will ever want to get within 50 feet of the car in front.
Years ago there was an after market brake light in the form of a miniature stop light. It had a line to the intake manifold (engine vacuum line) in addition to the electrical connections.
As long as you were going at relatively constant speed the light stayed green. If you let off the accelerator the light changed to yellow for caution, and when you put on the brakes the light changed to red.
I think this idea should be put into use sans the intake manifold connection. Ordinary tail lights should be yellow not red. There is nothing dangerous about a car going ahead of you at the speed of traffic but caution is warranted. Then when the brakes or turn signals are applied use a red light. The change in color would be more distinctive than a change in intensity. Those drivers with marginal color vision in the red-yellow wavelengths would be a problem but surely someone would be able to figure a way around that. Such drivers get along with traffic signals and emergency vehicle lights now.
David, as pointed out, that is one reason why the center brake only light was mandated. It only lights for braking, as opposed to the tail lights. Better than any color change is a specific location. Which is how colorblind can tell traffic lights apart.
I’ve heard that 1986 was the first model year of the CHMSLs, and my '86 Pontiac does indeed have one. However, I seem to remember that the '87 Plymouth Voyager that my mom used to drive did not have the CHMSL. Was the implementation delayed for trucks and vans, as auto safety laws often are, or am I not remembering correctly?
By the way, one of the crazy ideas I once had, and never tried to carry out, was to somehow rig up one of those “marquee” things that spell out words with red LEDs in the rear window of my wagon. I thought it would be cool if I could get it to display “BRAKE”, “ACCEL”, “<—LEFT”, and “RIGHT—>” at appropriate times, but the idea never got beyond the speculation stage for a variety of reasons. Oh well.
-Andrew L