I leave a lot of distance, but people rarely change lanes to get in front of me, because the person I'm following isn't going any faster; the separation is in d not v. Most people who do change in front of me are there only briefly on their way to an exit, which means I'm increasing the fluidity of the traffic flow, so I'm happy about that. If I didn't leave space, they'd likely hard-brake in front of someone, causing a slowdown.