Causes of Road Transportation Bottlenecks
Bottlenecks are major impediments to the free flow of traffic and occur under specific circumstances, mostly when nearing capacity. Under such circumstances, a small bottleneck can have important consequences, leading to an accumulation of delays. Four major causes can be identified:
  • Traffic interruption. By far the most common cause and include traffic lights, stop signs, tolls and railway crossings. When traffic lights are not well synchronized with the direction and flow of traffic, significant delays may occur while traffic accumulates in one direction. Tolls can also be a major bottleneck, especially in urban areas, as significant time can be spend waiting to pay the manually collected fare. The emergence of electronic toll systems has partially alleviated this issue.
  • Lane reduction. The merging required when the number of lanes is reduced can easily become a bottleneck, especially if the capacity on the segment becomes lower than the traffic on the previous segment. The unmet demand thus becomes traffic delays.
  • Merging. Although highways are designed to provide an uninterrupted flow to traffic, merging can be a cause for bottlenecks as cars are slowing down and changing lanes. This is notably the case at the intersection of two major highways where a large amount of traffic shifts from one highway to another.
  • Distraction. This type of bottleneck is created by a psychological reaction of drivers to an unusual event that, although does not directly influence the capacity, is distracting the traffic and causing a slowdown. This is often known as "rubber necking". Distractions can often be as trivial as a car that was pulled over by the police. Accidents on an highway often results in a bottleneck on the opposite lane as drivers slow down out of curiosity. Glare during sunrises and sunsets is also known to create bottlenecks over specific highway segments.