A 4 lane road just means more than twice the traffic of a 2 lane road. In heavy traffic, it isn't always easy for all of the cars in one lane to pull over into the other lane to pass a "slow moving vehicle", and the disruption to traffic flow remains, as well as a possible increased danger for the vehicles. And it is easy for cars to stack up behind an unexpected slowing.

The alert rider would be able to judge the amount of disruption to the traffic around himself or herself.