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.