It's actually pretty realistic for very lightly congested roads. At those levels of congestion, the primary bottleneck is the distance and speed-limit. Marginal

It's actually pretty realistic for very lightly congested roads. At those levels of congestion, the primary bottleneck is the distance and speed-limit. Marginally increased traffic will have minimal impact on travel time. In contrast, for heavily congested roads, the primary bottleneck is the congestion itself. Hence, marginally increased traffic will have a proportional impact on travel time.