The speed the car in front of you drives that is just fast enough to not pass them up, but not quite fast enough for you to be content. This often happens when you pass up a slow car, but they notice you and go faster. You then get back behind them and they are going just almost fast enough. This speed is often more angering than someone going too slow or too fast, because there isn’t really a good solution to it, as you cant easily pass them up without going too fast, but if you stay behind them you will be forced into going too slow.
“This car was going way too slow, but when I tried to pass it up, they sped up to mediocraspeed. I didn’t feel comfortable passing them up, but I really didn’t want to get stuck behind them. It was a terrible situation.”