When riding your bicycle down the road, you should always ride with traffic (except where local laws state otherwise) because it is much safer for you.
You want to ride with traffic because…
- …in the US it is illegal to ride opposing traffic.
- …you are more likely to escape collisions without a serious injury.
- …if everybody did this, then you would not have to worry about passing somebody going in the other direction while two cars are also vying for road real estate at the same time.
- …drivers expect you to be riding along with them and predictability makes you safer.
- …drivers would often not have enough time to react to your being on their side of the road if you were both going at each other with any sort of speed.
When I plan my routes, I try to make them go in a (generally) clockwise direction so that I will be riding with traffic (at least, on this side of the pond). When I can find trails, I prefer to take them as it takes traffic directly out of the equation.
I rarely ride my bicycle on a sidewalk and much prefer the road itself. If you are on a trail, then you should also ride on the right side. This will make it less confusing if you are passing cyclists, rollerbladers, or pedestrians. Again, predictability will make you safer.