Civil Engineering Reference
In-Depth Information
c
congestion problem. Travelers who live in low-density suburban areas, where trip
lengths are longer, will be penalized more by congestion than those travelers who
live in more compact urban areas where trip lengths are shorter. A delay rate of
1 min per mile would add 15 min to a 15 mile trip, but only 5 min to a 5 mile
trip. Therefore trip length should be included when analyzing the effect of con-
gestion on travelers.
Because average trip length varies with population density (Fig.
9.2
), land use
density should be considered as a potential mitigating factor in reducing the effects
on congestion travel. But to consider changing land use patterns as a mitigating
factor makes it necessary to change the perspective of congestion
Therefore, land use patterns and area size play a key role in analyzing the traf
—
from one
focused primarily on traf
c speed
to one that also includes
trip time
.
9.3 Travel Time Reliability
Travel time reliability is often viewed as more important than average travel time.
The ability to predict travel time is highly valued by travelers and the business
community. Travel time variability affects trip starting time, the choice of routes,
and travel modes. This condition requires travelers and freight carriers to add a time
buffer in planning and scheduling their trips. A route that takes a longer average trip
time but experiences a smaller variability in travel time may be preferred to one
with a shorter average travel time but with a larger travel time variability.
Travel time reliability is a key performance indicator of traf
c congestion. It can
be de
c
time. While other delay metrics quantify congestion using average values (for a
speci
ned as the degree of certainty that a trip will be completed within a speci
c time period) reliability metrics focus on the ranges and distributions of
travel times that are not likely to be exceeded
—
usually the 80 and 95 percentiles.
9.3.1 Sources of Travel Time Variability
Sources of travel time variability include the following:
Travel Demand Volume
—
travel time varies with traf
c demand on the road-
way: more traf
c increases travel time and less traf
c reduces it
Traf
c incidents
such as crashes and debris on roadways block travel lanes, and
increase travel times
Work zones
reduce vehicle speeds
Environmental conditions
such as inclement weather that reduce vehicle speeds
and increase vehicle spacing
