If you sit in a traffic jam or just make a stop at the traffic lights, close your eyes with your hands for 20-30 seconds, take a deep breath, relax, think about something nice. It will help your eyes to put the road aside and take a rest for a while. You can gently clap the face with your fingertips.
There is another effective exercise helping to reduce eye strain: Focus on any thing which is closely spaced and hold it about 3-4 seconds, then turn your glaze abruptly to a distant object. Repeat it for 5-6 times. The main thing is not to give your passengeres a fright, because they may decide that you fall asleep, staring at the wheel.