Towing speed

One thing about Interstate highway travel is you rarely if ever find someone driving at the exact speed you want to drive. You inevitably come across someone who is poking along just slightly slower than you are, and need to pass. As soon as you pull out to pass, that person matches speed with you so you've either got to punch it to bet past, or fall back in behind. If you pass, they zoom on by, slip in in front of you, and slow down. If you fall in behind, they slow down.

In short, your speed is often dictated by the jerk in the right lane who is either being a jerk on purpose or is such a sheep that they set their speed by what they see in their peripheral vision.