Running along the roads, near heavy automobile traffic, adds the risk of being struck by a vehicle and also exposure to air pollution emitted by those vehicles. Ground-level ozone, particulates, and hydrocarbons. All that damages lung tissue and cause a variety of respiratory problems.
It is very important to avoid that air pollution in general, and essential to keep an eye on traffic to avoid being hit. Even though it is customary and, in most places, defined by law in America that bikers and pedestrians travel in the same direction of traffic, I can’t get myself to do it. I prefer to travel against traffic so I can keep an eye on it and will always have a chance to jump out of the way if a distracted driver gets too close. With the traffic coming from behind I would never have that chance.