This may seem like a silly topic, but one of the skills taught in the League of American Bicyclists (LAB) is the “Quick Stop.”
Most of the time, simply squeezing the brake levers (on most bikes) will slow and stop you adequately. There are times, though, when you need to stop on a dime. Knowing how to quickly stop a bike — and practicing these steps — can save you from a tacoed wheel or worse. It’s even conceivable that a very steep hill might negate much of the stopping power that a rear-only brake can provide.
Most of the braking power on a bike comes from braking the front wheel. Yes, its possible to lock up the front wheel and lose control, which is why practicing before you need the panic stop is important. Maximum braking occurs at the point right before the rear wheel leaves the ground; at this point the rear wheel provides no stopping traction so it’s a little pointless to apply the rear brake.
Back to the quick stop. If I recall, I think the LAB teaches cyclists to apply about three times more force to the front brake than the rear. Many cyclists, including myself, are able to maintain control even when the rear wheel leaves the ground. If you feel that happen, you relax the braking pressure a little to get that rear wheel back down. One of the reasons practice is important is so you don’t freak out and freeze up and lose control when this happens.
There are exceptions, of course. Don’t use the front brake exclusively in slick or wet or any other conditions where the front tire might slide out while braking. You should slow down to account for the increased stopping distance. I’m assuming that most commuters ride on paved surfaces — this advice is completely inapplicable for mountain bikers.
“What if my brakes fail?” Well, then, you’re hosed. Some things to try, though, are jamming a shoe into the space between the downtube and front tire, being careful not to lock that tire up. Another trick that works is to pivot your heel inward so it rubs against the rear tire. That probably won’t get you stopped, but it may slow you enough so that you can bail. Another trick is the “Fred Flintstone” — drop down onto top tube and skid your feet on the ground. If you’re headed toward traffic or other obstacle and you’re not getting stopped, your only option may be to bail out. Hop backwards off of your saddle and land on your feet running, hoping you won’t fall in the process.
A word about maintenance: Ensure your stopping surface (usually the rims on commuter bikes) are in decent condition. If they’re scored from overuse, it’s probably timme to replace them. Brake pads are expendables and should be replaced when they’re worn down. Check brake cables or hydraulic lines and the levers for wear, also.