Measuring Traffic Congestion - Road Traffic Congestion - page 87

Table 8.1 Overview of congestion indicators and their metrics
Congestion aspect System type

Single roadway

Corridor Area wide network

Duration (e.g.,
amount of time
system is
congested)

Hours facility
operates below
acceptable speed

Hours facility
operates below
acceptable speed

Set of travel time contour
maps; “ bandwidth ” maps
showing amount of
congested time for system
sections

Extent (e.g.,
number of people
affected or
geographic
distribution)

% or amount of
congested VMT or
PMT; % or lane-miles
of congested road

% of VMT or PMT in
congestion; % or
miles of congested
road

% of trips in congestion:
person-miles or person-hours
of congestion; % or lane-
miles of congested road

Intensity (e.g..
level or total
amount of
congestion)

Travel rate; delay rate;
relative delay rate;
minute-miles;
lane-mile hours

Average speed or
travel rate; delay per
PMT; delay ratio

Accessibility; total delay in
person-hours; delay per
person; delay per PMT

Reliability (e.g.,
variation in the
amount of
congestion)

Average travel rate or
speed ± standard
deviation;
delay ± standard
deviation

Average travel rate or
speed ± standard
deviation;
delay ± standard
deviation

Travel time contour maps
with variation lines; average
travel/time ± standard
deviation; delay ± standard
deviation

Note VMT vehicle-miles of travel
PMT person-miles of travel
Source Reference [ 7 ], NCHRP 398, Vol. 1, Table S-5, p 7

Fig. 8.1 Intensity of
congestion in time and space.
Source Reference [ 6 ],
NCHRP 398, Vol. 1,
Figure 14, p 70

Congestion Delay Rate ¼½ Congested travel time rate ð min/mi Þ

½ Uncongested travel time rate ð min/mi Þ

ð 8 : 1 Þ

¼½

Þ ½ 1 = congested speed ð mph 1 = free flow speed ð mph Þ

ð 60 min = h Þ