Figure 2.60 shows a graph illustrating what happens when demand exceeds capacity.
The straight line represents the capacity of a section of freeway at a particular time (i.e.,
the number of vehicles getting past the point under prevailing roadway conditions). As
long as traffic demand, or the number of vehicles arriving at that point (shown by the
curved cumulative demand line), is less than or equal to the capacity of that section of
the freeway, there is little congestion. However, once the arrival rate begins to exceed the
capacity at time Ta, a bottleneck is formed and vehicles begin to accumulate upstream
until time Tb, when the demand once again falls below the capacity. Congested conditions
continue until time Tc, when the accumulated traffic at the bottleneck dissipates. The area
between the capacity and demand curves during congested conditions is the delay resulting
from the congestion (Ref. 4).