California drivers do tend to not “keep right”, partly because the highway designs that Caltrans uses tend to prefer angled-merge on-ramps rather than parallel

California drivers do tend to not “keep right”, partly because the highway designs that Caltrans uses tend to prefer angled-merge on-ramps rather than parallel acceleration lanes. Drivers soon learn that the rightmost lane often has congestion due to merging cars arriving at the merge point in groups created by off-highway traffic lights, and then tend to avoid driving in the right-most lane. Metering lights on the on-ramps help reduce the grouping, but the angled merges remain, as does the learned behavior. Also, many US states post signs: “Keep right except to pass” but California posts “Slower traffic keep right” and since nobody wants to identify as “slower” we do not. Interstate 5 in the Central Valley is generally two lanes, and semi-trucks and vehicles pulling trailers are limited to 55 mph, and must keep right except to pass. Other traffic has a higher speed limit. When traffic is heavy, nobody leaves the left lane because they are much less likely to find a gap to merge into.