Office of Operations
photos of traffic merging onto congested highway, congestion in snowstorm, variable message sign, cargo, variable speed limit sign in a work zone, and a freeway at night
21st Century Operations Using 21st Century Technologies

Traffic Bottlenecks

Confluence vs Slowing

Not all bottlenecks – either recurring or nonrecurring – are the result of a "bottle neck" design; i.e., a funnel-type constriction like a lane drop or an accident taking away a lane. Some recurring bottlenecks are not in any way complicit with a reduction in capacity. Some result from confluence (e.g., mixing) and some from slowing (e.g., rubbernecking).

A confluence is generally defined as a coming together, a meeting, or mixing. Using classic examples, this can be a forced merge, as in the case of a lane drop, but also a mixing, as in the case of a weave condition. In the first case there is a loss of capacity. In the second case, there is no loss of capacity but in either case a confluence results.

Examples of Confluence

  • Weave
  • Forced merge (lane drop)
  • Highway off-ramp followed by on-ramp
  • Necessarily crossing lanes to get into the correct turn lane

A bottleneck caused by slowing is one in which motorists more or less do not have to change or fight for lanes, but are effected by mostly-superficial impacts that simply cause them to hit the brakes, and cause motorists behind them to react.

Image of a driver facing sun glare.
A driver facing sun glare

Examples of Slowing

  • Rubbernecking
  • Sun glare
  • Dusk
  • Distracted driving
  • Wet pavement

Sun glare, dusk and wet pavement, et al, could easily be viewed as weather-related nonrecurring events. However, in the context of facing these conditions rather frequently (or in the context of some academics questioning if these aren't in fact nominal occurrences) can one justify that these are normal conditions that drivers should have some expectation and familiarity with, which is to say, should the mere use of headlights or windshield wipers really be the cause of congestion? And we are not speaking of atypical reactions, fears or even phobias. Psychologists and human factors experts remind us that categorical phobias exist; for example, the mere "fear of driving" (a recognized stress disorder with no Latin term per se but comparable to 'hodophobia', the fear of traveling) or the fear of driving across bridges ('gephyrophobia'), etc., and other phobic extremes, like rain-on-windshields, or driving next to jersey walls.