Congestion is usually cited as the illness of urban transportation. Other segments of the transport supply scene suffer from the same malady. Where demand-capacity ratios approach or exceed one, congestion occurs. A form of congestion happens when demands are concentrated in short periods of time. This is the RUSH HOUR type of phenomenon. Congestion is difficult to quantify even though the implications of congestion are generally understood.
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An objective measure of congestion is the time delay during congested conditions in comparison with ideal. Time delay caused by demand behaves differently for different types of transport service. Scheduled services for instances are intended to experience no time delay even when the flows are at maximum. These systems have time delays experienced by customers when the demand is less than maximum. Highway drivers and many other types of transport users experience the time delays as the demands are high.
Only in very rare cases is the traffic demand constant or inelastic so that a capacity increase is very often accompanied by a demand increase. When there is a large deficiency in capacity the demand may appear almost perfectly elastic and any new capacity is immediately utilized even though it was expected to fulfill the demand (need) for some time in the future.
Examples of this sort of situation are very common and indicate that some feature of the demand is not understood by either the forecasters or the critics. With some notable exceptions a highly elastic type of demand situation exists for many transportation situations especially in terms of time and in relation to urban areas.
This is due to a number of factors. The most important is the effect of deadline demands (i.e. demands that are essentially instantaneous). The second is the diversion of portions of traffic from situations of higher to lower states of congestion.
Capacity also is not a fixed phenomenon in most transport systems. It depends on the acceptable minimum level of service which seems to increase with time and experience.
The best measures of urban congestion effects are measures of trip time, i.e. total movement time from origin to destination. This measure is probably more acceptable as a measure of congestion for person trips, but most will agree that the first signs of congestion for goods movements will be delays to normal shipment schedules.
Reductions in Congestion are improvements in trip time that are consistent with ability to move the required amount of people and or goods as close to optimal timing as practical.