Civil Engineering Reference
In-Depth Information
application of ITS technology, and (2) adding new capacity (e.g., removing geo-
metric bottlenecks by widening roadways, adding lanes, or building new road-
ways). These strategies are described in Chaps.
15
-
c demand consists of strategies that reduce automobile use or that
offer mobility alternatives to private motor vehicles. These include: congestion
pricing, road pricing, parking pricing, regulatory restrictions on auto use, employer-
based programs, improved public transportation, and land use patterns that
encourage alternatives to auto travel. These strategies are described in Chaps.
18
Reducing traf
23
.
-
14.3.1 Overview
Reducing congestion in a growing metropolitan area is typically achieved by
implementing a combination of strategies that maximize available capacity, add
new capacity, reduce automobile use, and reduce the growth in motor vehicle
traf
c. These strategies might involve increasing the choice in the number of routes
and travel modes, or the creating traf
c lanes that involve the payment of a toll for
higher-speed and higher travel time reliability.
Table
14.1
gives the nine possible states of capacity and/or traf
c demand that
impact on traffic congestion.
The capacity-demand states (0
8) identi
ed in Table
14.1
are brie
fl
y summa-
-
rized below using Fig.
14.1
.
0 Existing condition: This state denotes the congestion level being experienced
and serves as a reference point from which the bene
ts and costs of changes in
either roadway capacity, traf
c demand, or both, can be evaluated.
1 Traffic demand increases (D+) and capacity remains the same (So):By
keeping capacity constant (So), an increase in traf
c demand over time (D+)
increases congestion (lower speed) and increases vehicle-miles traveled (VMT). An
example of this case is where population, car ownership, and economic activities
grow while roadway capacity does not change. This condition prevails in large
urban areas where transportation investment levels have not kept pace with demand
growth. Demand mitigation strategies are most appropriate in large urban areas
where roadway capacity increases are least feasible.
2 Traffic demand decreases (D
−
) and capacity remains the same (So):By
keeping capacity constant (So), a decrease in traf
c demand over time (D
)
−
Table 14.1 Possible
combination of traffic demand
and roadway capacity
strategies
Traf
c Demand
Roadway capacity
Increase
Same
Decrease
Increase
5
1
6
Same
3
0
4
Decrease
7
2
8
