Why do bicycles have two individual brake levers that work independently to brake each wheel? I've always used my right brake lever because it is more comfortable to do so and I haven't noticed any difference between the two.
I see several reasons:
- Redundancy of an essential safety feature is good. If there's a problem with one brake lever you still have the other brake. Being totally unable to stop could be disastrous.
- It is a legal requirement in some places to have two independent brakes for a bicycle used on public roads.
- Limited human hand strength. One hand can't pull the brakes as hard as two. If you need to stop really quickly this could make a difference.
- Separate control of braking. The rear brake is much less effective and prone to skidding. The front brake is more effective for quick stops, but locking up (skidding) the front wheel will lead to a crash. @Stephen Touset wrote up a detailed answer covering the control issues.
- Allows signaling with either hand and stopping at the same time. Sometimes you need to be slowing down to prepare for a stop or turn and signaling at the same time. If you can take one hand or the other (but not both) off the handlebars that can be useful. (I prefer the arm straight out in direction of turn signal instead of the left arm hooked up to signal right turn)
- Simplicity of design. A single lever controlling both brakes (via two cables?) would be tricky to adjust right, and if it wasn't adjusted exactly perfectly you'd probably end up braking with one wheel and the other never really happening. The longer cable (rear brake) would get more stretch (housing compression) just because it's longer, so you'd probably need to be fiddling with barrel adjusters on that cable quite frequently. One lever controlling both brakes just seems difficult to engineer right and likely to need much more frequent adjusting/maintenance.
Personally, I tend to use the rear brake for controlling my speed (slowing down just a little) and either the front brake or both brakes for stopping (especially a fast stop). An awareness at any given time of which tire is more likely to slip and using the other brake is good, though.
Worth noting: it's a specialty item, but you can get a brake lever that controls both brake levers at once. This is mostly popular with bike polo players (who use one hand for a mallet) and people with physical conditions that limit their ability to use the brake with one hand. If you don't have any special reasons to need just one lever, you're probably best off with separate levers. http://www.paulcomp.com/duplexlever.html
Linking the two brakes would have a detrimental effect on braking power.
Your front brake will bring your bike to a halt far quicker than your rear brake will, and should be used almost exclusively. When braking with the rear brake, your back tire won't have much weight on it, and will skid along the ground. This results in a dramatic reduction of braking power. On the other hand, when you compress your front brake, the forward momentum of the bike will be partially converted to downwards force, sticking the tire hard against the road surface and maintaining maximum traction. In a real-life scenario, this can easily be the difference between slamming into a car that suddenly brakes in front of you versus screeching to a halt with an inch to spare.
Of course, if you squeeze the front brake too hard at too great a speed, you can flip over the handlebars. However, if you primarily use your front brake, you will quickly learn exactly how quickly you can safely stop the bicycle. Being low in the drops helps with this immensely.
The few scenarios in which you should be using your rear brake are during fast downhill descents, where even a little braking on the front at fast speeds can cause your center of gravity to go over the front of the bike, or on any surface where your front wheel might lose contact with the ground. In this situation, the front brake could bring your front wheel to a complete stop just before it connects with the ground again, causing you to lose control.
It's about security that (at least) two completely independent break mechanisms should exist. If one mechanism fails, the other are there to serve. In competitions and other special circumstances like high speed, slippery roads and down hills, you also are helped with choose front / rear depends on the situation.
Individual brake levers for each wheel give you more options for braking. There are some circumstances when you would only want to apply one brake - e.g. if you get a puncture.
I note that no one has mentioned that you want to have separate control over front and rear to prevent flipping. [Well, on re-reading, I see that Stephen did.] The front brake carries about 80% of the braking power, but if it locks up the bike flips rather quickly. If the rear brake locks up the bike will tend to slide sideways. When attempting to stop as rapidly as possible one should apply the most force (2-3x) to the front brake but then, if the rear wheel begins to slip, release the FRONT brake. (The rear wheel will slip because it's getting "lighter" due to the front brake being near lock-up.)
It's fairly simple; to stop effectively, you need to apply the two controls in a different manner.
When you start braking, you have a given amount of weight on the front and rear wheels. As you apply the brakes, the deceleration shifts weight from the rear wheels to the front wheels, which gives more traction at the front, and less traction at the rear. More traction at the front means that you can apply the brakes more there, less traction at the rear means that you need to reduce the power on the rear brake.
When I used to teach motorcycle safety, we would say "progressive squeeze on the front brake, light to lighter pressure on the rear".