You are driving a vehicle that could be driven safely at 55 mph on an open road. But traffic is heavy and other vehicles drive at the speed of 35 mph, though the speed limit is 55 mph. The safest speed for your vehicle is more likely to be:
When you are driving in heavy traffic, the safest speed is the speed of other vehicles. Vehicles going the same direction at the same speed are not likely to run into one another.