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. The most common cause includes 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 spent waiting to pay if the fare is manually collected. The emergence of electronic toll systems has alleviated this issue.
- Lane reduction. The merging required when the number of lanes is reduced can easily become a bottleneck, especially if the capacity in the segment becomes lower than the traffic in the previous segment. The unmet demand thus becomes traffic delays.
- Merging. Although highways are designed to provide an uninterrupted flow of traffic, merging can cause bottlenecks as cars slow down and change 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 it does not directly influence the capacity, is distracting the traffic and causing a slowdown. This is often known as “rubbernecking”. Distractions can often be as trivial as a car that was pulled over by the police. Accidents on a highway often result 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.