The Costs and Other Consequences of Traffic Congestion - Road Traffic Congestion - page 151

Fig. 13.1 Averages do not tell the full story. Source Reference [ 12 ], Fig. 1

For example, a traveler on congested roads who experiences variable travel
times on the commute to work needs to allow for this variability by starting the
trip suf ciently earlier to ensure on-time arrival most of the time.

However, since travel time variability data are not typically measured, they
generally are not included by transportation agencies in their calculation of trip
times and costs. This is potentially a large underestimate when one considers
that in large urban areas between 58 and 67 % of congestion delay on US arterial
roads is attributable to random events a major source of travel time variability

(see Table 7.2 ) . Moreover, unexpected delay time is reported at a value of over
2.5 times the value of travel time under normal conditions [ 2 ].
c. Travel Time Reliability Impacts on Freight Movement [ 13 ]
The reliability of the highway system in enabling goods to get where they need
to be when they need to be there is a key performance indicator because
“

—

Unexpected delays can increase the cost of transporting goods by

50 250 % ”

- [ 14 ].

“

u-
encing choices in freight transportation. A 1999 study estimated that carriers on
average value savings in transit time at $144

Studies have shown that reliability is one of the most important factors in fl

193 an hour and savings in
schedule delay at $371 an hour [ 15 ]. As in the case of automobile travelers,
truckers value time savings in congested conditions more than twice as highly as
overall travel time savings.

-

” [ 2 ].

13.2.1.2 Value of Travel Time

The valuation of travel time reported in the literature represents average values that
were aggregated across all travelers and conditions. They are not intended to rep-
resent the travel time values experienced by individual travelers. Since the effect of
travel time reliability is rarely considered in project assessments, proposed trans-
portation projects that improve travel time reliability may be undervalued [ 2 ].

Person Travel
Table 13.1 gives the best single ning the value of travel time savings

(VTTS) as a percentage of hourly income. Table 13.2 summarizes a likely range of
travel time values for each trip category. The ranges are not necessarily symmetrical

figures for de