Civil Engineering Reference
In-Depth Information
Comment
:
In large urban areas, using a free-
ow delay rate as a reference to measure delay
overstates the time lost because of congestion.
fl
8.2.1.3 Person-Hours of Delay
Person-hours of delay are computed by applying vehicle occupancy factors to the
observed vehicle data.
Annual Person Hours of Delay
¼½
ðDVHD=weekdayÞxð
250
weekdays=yearÞ
x
ð
1
:
25
persons
=
vehicle
Þ
¼½
ð
Annual Hours of Delay per Weekday Traveler
Actual Weekday Travel Time
;
in minutes
Þ
ð
FF or PSL Travel Time
;
in minutes
Þ
½
1h
=
60 min
¼
Delay Hours per person; per year
½
250
weekdays per year
ð
8
:
3
Þ
where:
FF
ow speed
PSL = posted speed limit speed
= free
fl
Comment
:
The delay data calculated by Eq.
8.3
are widely used by a variety of sources
including the Secretary of the US Department of Transportation in his testimony to
the Senate Committee on Housing, Banking, and Urban Affairs, [
13
]. The data are
widely disseminated by the national press each time they are annually updated.
A 2011 headline on the Wall Street Journal [
14
] states
“
Chicago and Washington,
DC drivers idle for an average of 70 h a year in traf
c jams, according to the latest
Urban Mobility Report from the Texas Transportation Institute.
”
This headline obviously exaggerates the time lost in congestion
—
unless one
believes that in large urban areas it is possible to travel at free-
fl
ow speed during the
am and pm peak hours.
8.2.1.4 Travel Time Index (TTI)
The travel time index developed by the Texas Transportation Institute (TTI),
compares the travel time rates in the peak period, to travel time rates during free-
fl
flow or posted speed limits [
10
]. The TTI is calculated as shown below:
