Using traditional yearly average congestion estimates can
obscure important variations:
• changes in congestion taking place during the year,
• considerable differences in travel between the morning
and afternoon periods, and
• differences in congestion patterns for weekday and weekend
travel.
An examination of monthly congestion data, in particular,
could offer a view of the systematic intra-year movement or
seasonal patterns of congestion.
The average monthly number of weekday congested hours
per day for Chicago, Los Angeles, and Houston varied substantially
among the three cities over the 3-year period from
April 2004 to April 2007 (fi gure 1).5 The total number of congested
hours per day for each city should not be compared to
the other cities because the data collection system coverage
and free-fl ow speeds for each city vary dramatically.
However, it is useful to look at the trends and variations
in those data. There are some similarities in the trends for
the three cities; for example, a tendency for lower levels of
congestion near the end of each year. Because seasonality
is often driven by periodic fl uctuations in weather, vacations,
or holidays, the declining congestion around December is
not surprising. But there are variations in these patterns, both
over time and among the three cities.