The sign warns trucks that there is a 7% grade for the next 8 miles. No problem. You shift into a higher gear, lean into the drops and start pedaling faster. Going downhill on a road bike is a combination of comfort level, knowing what your bike can handle and employing a few standard techniques to keep you safe.
Comfort Level and Cycling Ability: Some people get going fast and they tense up; others get going fast and they only want to go faster. It's a comfort-level thing. Your comfort level is determined somewhat by genetics (some are just wired to go fast), but mostly by practice and confidence. The surer you feel about your bike and your ability to handle it, the more comfortable you're going to feel about the speeds you reach as you fly down a hill.
Your Tires: Traction is generally not an issue when on a road bike. And advances in tires have made them more reliable than ever. However, staying vigilant and scanning the road for debris can help make your ride trouble free. What do you do if you encounter poor traction?
Sand or Gravel:
Avoid hitting your brakes too hard when you encounter poor traction as you could become unbalanced while going through the rough. This can end in a crash. If you determine that you have time to brake, throw your weight back and do so. But make sure your weight comes forward and that your feet are in the platform position when you reach the rough. Make sure your body is ready to absorb shock. Keep a firm but relaxed grip on the bars and don't touch the brakes once in the area of poor traction. Also, resist the urge to put a foot out for stability. This is a sure way to crash. Think light and imagine floating over the rough. The key is to keep balanced and stable. This will see you safely through. You can scrub speed if you need to after the sand or gravel is safely behind you.
Wires, boxes, soda cans and more: This is the garbage with which road bikes must contend. The best defense is a good offense where road debris is concerned. Keep your eyes scanning ahead and looking for debris. It's easy, especially on long stretches, to become mesmerized by the speed and the rushing wind. Don't! However, if you find debris in your path, the best you can do is a slight bunnyhop (if you feel comfortable doing bunnyhops), or to run the debris over. Either way, a relaxed grip and coming up slightly out of the saddle will help. If the debris does cause you to crash, remember to stay loose and roll with the crash. Putting your hands out will cause more harm than good.
Knowing when not to use the brakes is perhaps the most important thing to know about descending on a road bike. Try to keep your hands off the brakes. Why? Even slight use of the brakes at the inappropriate time can throw off your balance or take you off the line you've established. If you do use them, keep the pressure light, move your weight back and apply both brakes evenly.
As with a mountain bike, the steeper the hill, the more your weight should be back on the saddle. Pressure on the rear wheel helps with braking and with cornering. Make sure your grip is relaxed and get into the drops with your hands near the brakes. Flying down a hill is not the proper time to be casually steering your bike from the brake hoods.
When cornering, remember to drive down the outside leg and lean on the inside drop. If you need to throw a knee out, that's fine, but keep your outside elbow in close and your head and shoulders level. This will help ensure your tires stay in contact with the road.
The Basic Idea: If used in unison, all of these techniques will result in a safer and more enjoyable ride. With practice and development of the proper muscles and reflexes, balance and control become second nature. Then you can concentrate on the simple merits of speed—a good, tasty downhill, the wind rushing through your helmet, the exhilarating blur of scenery.