Traffic Congestion and Reliability: Linking Solutions to
Problems 2.0 Background to the Congestion Problem Congestion
results from the interaction of many different factors — or
sources of congestion. Congestion has several root causes that
can be broken down into two main categories: Too much traffic
for the available physical capacity to handle – Just like a pipe
carrying water supply or the electrical grid, there are only so
many vehicles that can be moved on a roadway for a given time.
Transportation engineers refer to this as the physical capacity
of the highway system. Physical capacity is determined by such
things as: how many lanes are available to carry traffic, the
curvature of the highway, side clearance, and interchange and
intersection design (for example, length and position of on-
ramps and exclusive turning lanes at intersections). Physical
bottlenecks are locations where the physical capacity is
restricted, with flows from upstream sections (with higher
capacities) being funneled into them. Th