Keeping constant spacing and running at a reasonable speed within a lane may be good. But holding the same speed in adjacent same-direction lanes is very bad.
In driving classes, back in the mid-20th century, we were warned against it. You NEVER were to hold the same speed as a car in an adjacent lane. (About a 5 MPH drift, with leftward lanes faster, was close to ideal.) Judging by the behavior of current drivers on California freeways that lore has apparently been lost.
Some of the issues:
- Adjacent cars form a multi-lane "rolling roadblock". Drivers behind them who wish to travel faster are impeded, collect behind them, and end up "compressed", setting up the conditions for a chain, reaction multicar pileup.
- With an inter-lane drift a driver wishing to pass a slower car soon has an opening to switch lanes and proceed.
- With the slowest lane to the right and increasing speed to the left, merges and exits require less speed change and have better timing margins, long-distance traffic proceeds rapidly with little disturbance, and lane changes are easy. Drivers have the opportunity to rapidly distribute themselves among the lanes and drive at a speed where they're comfortable.
- When driving at the same speed as an adjacent vehicle you increase your risk of collision:
- If you're in a blind spot you STAY in the blind spot for a long time. The window of opportunity for the adjacent driver to happen to make a lane change into you - or into the space immediately in front of you, becomes much larger than if you had a relative drift.
- If you hold relative position the other driver's peripheral-vision motion detector doesn't keep him aware of your presence. After a minute or so you're likely to fall out of his attention. Then, if a sudden traffic situation makes him need to change lanes suddenly (or he just wants to change lanes and forgets to do a recheck), he may swerve into you.
(By the way: The two-way two-lane equivalent of the rolling road-block chain-reaction-collision precursor is the "rat pack", a term of art in traffic engineering. It occurs when the first driver goes slightly over the limit and the second driver won't pass because he doesn't want to risk the necessary speed, but follows too closely for following cars to pass in two single-car hops. Fault is primarily on the second driver.)