Face it, there are distracted drivers everywhere. All you can do is be alert, leave enough room between your car and the car in front of you so you have time to react if need be. You just do what you can to obey the laws and safely get to where you are going.
It is when you ALLOW someone elses driving habits to effect your mood or cause anger, leading to possible road rage conditions, is when the situation can get out of control. So, just maintain your cool and move on without incident.
The speed limit is set for road conditions. By the way, going under the speed limit is not such a horrible thing. It is up to the drivers to be alert and drive safely, under all conditions and speeds, regardless of the posted speed limit. Our highways have 65mph signs on them with a sign that says "Minimum Speed 40MPH". That is a 25 mile an hour difference! So, surely there is leeway for 10 mph below the speed limit.
Some people can't drive. That is for sure. But, as a responsible driver, it is your responsibility to be alert and avoid interacting with bad drivers. Your temper is important to quell so there are no confrontations with strangers with whom you no idea how will react. Stay safe.