"You're riding the wrong way (against traffic, on the left-hand side of the street). A car makes a right turn from a side street, driveway, or parking lot, right into you. They didn't see you because they were looking for traffic only on their left, not on their right. They had no reason to expect that someone would be coming at them from the wrong direction.
Even worse, you could be hit by a car on the same road coming at you from straight ahead of you. They had less time to see you and take evasive action because they're approaching you faster than normal (because you're going towards them rather than away from them). And if they hit you, it's going to be much more forceful impact, for the same reason. (Both your and their velocities are combined.)
Riding against traffic may seem like a good idea because you can see the cars that are passing you, but it's not. Here's why:
1. Cars which pull out of driveways, parking lots, and cross streets (ahead of you and to the left), which are making a right onto your street, aren't expecting traffic to be coming at them from the wrong way. They won't see you, and they'll plow right into you.
2. How the heck are you going to make a right turn?
3. Cars will approach you at a much higher relative speed. If you're going 15mph, then a car passing you from behind doing 35 approaches you at a speed of only 20 (35-15). But if you're on the wrong side of the road, then the car approaches you at 50 (35+15), which is more than twice as fast! Since they're approaching you faster, both you and the driver have lots less time to react. And if a collision does occur, it's going to be ten times worse.
4. Riding the wrong way is illegal and you can get ticketed for it.
One study showed that riding the wrong way was three times as dangerous as riding the right way, and for kids, the risk is seven times greater. (source)
Nearly one-fourth of crashes involve cyclists riding the wrong way. (source) Some readers have challenged this, saying if 25% of crashes are from going the wrong way, then riding the right way is more dangerous because it accounts for 75% of crashes. That thinking is wrong. First off, only 8% of cyclists ride the wrong way, yet nearly 25% of them get hit -- meaning wrong-way cyclists really are three times more likely to get hit than those who ride the proper way. Second, the problem with wrong-way biking is that it promotes crashes, while right-way biking does not. For example, cyclists running stop signs or red lights is 17% of their crashes. (source) But do we therefore conclude that not running signals causes 83% of crashes?!"
