Travel Flow Data
Some Basic Concepts
Good travel flow data for all modes of travel is important for
transportation planning and design. For example,
vehicle traffic flow is one of the data sets used to
estimate carbon emissions from transportation
activities.
One of the challenges is that travel flow varies significantly
in both space and time. We often do not have data at a
fine enough resolution to fully capture these variations.
It is important to understand the likely variations in order
to effectively interpreter the available data
Norman W. Garrick
Temporal Variation in Traffic Flow
Traffic flow vary by time of day, day of week, month of year
and from year to year.
The pattern of variation depends on the specific location.
For example, the temporal variation of traffic in Storrs is
very different from that in Willimantic.
Monday
Saturday
to Thursday
http://www.ptt.uni-duisburg.de/en/projekte/babnrw/daten/
Norman W. Garrick
Temporal Variation in Traffic Flow
One solution that is some times used to reduce temporal
variation is differential pricing.
For example, many transit systems charge a higher rate for
travel before 10 am and after 3 pm. This helps
encourage people that have flexible plans to delay their
travel to the off peak time.
This is the same approach used on some toll roads where
the plan is know as congestion pricing.
Norman W. Garrick
Spatial Variation in Traffic Flow
http://www.interstate-guide.com/images/i-077_va_aadt.gif
Norman W. Garrick
Spatial Variation in Traffic Flow
Directional Variation
Reversible lanes in the middle to deal with a severe directional variation.
(I believe this is just conceptual – I don’t know of any example of reversible lanes implemented in this manner.
Norman W. Garrick
Directional Distribution
In many urban areas trips are mostly going towards the
central business district in the morning and from the
CBD in the evening. This means that the trains and the
roads are sized to carry the peak direction flow. If the
directional distribution is very lopsided then this is a
very inefficient system since the lanes and the trains
going away from the center will be virtually empty in the
morning.
One argument for mixed land use is that it helps to cut
down on this directional over balance. So if a train is
connect two mixed use centers (such as downtown DC
and Arlington, Virginia) the directional distribution will be
more balanced resulting in more efficient use of the
transportation system.
Norman W. Garrick
Where does travel flow counts come from?
State Counts of Vehicle Traffic
The DOT maintain a program for counting traffic on all state owned
highways, roads and streets.
There are two different type of counts: permanent count stations and
temporary count stations
Permanent Count Stations give the most complete coverage of temporal
variation in traffic
Temporary count stations are much less reliable since they are put out for
at most 48 hours – the state then use factors to estimate the average
daily count.
Other source of traffic count data are counts from individual towns or from
developers working on larger projects.
Norman W. Garrick
Where does travel flow counts come from?
Pedestrian, Bikes and Transit Counts
I know of no agencies that routinely count pedestrian traffic – this makes it
harder to include pedestrian issues in transportation planning
A handful of cities in the country, including Portland, Davis and Cambridge
have programs for counting bike traffic
Transit counts are readily available from transit agencies and national
transit bodies
Norman W. Garrick
Portland (OR) Bike Count Program
Cyclists per Day
Bikeway Miles
1991
http://www.streetsblog.org/wp-content/uploads/2007/09/portland_bike_counts.jpg
2007
Biking, Davis (CA)
Norman W. Garrick
Shared Bikes, Paris
Norman W. Garrick
Bike Parking at Train Station, Amsterdam
Norman W. Garrick
Light Rail, Portland (OR)
Norman W. Garrick
What is the State Traffic Counts Used for?
AADT
The state traffic count is used to estimate an average annual daily traffic
(AADT)
The AADT is meant to represent the average traffic over all 365 days in the
year. In other words, it is meant to be
Total Traffic in year / 365
This can be obtained relatively accurately from the permanent count
stations.
From the temporary stations, this is more difficult. The count from the
station (which is referred to as average daily traffic or ADT) is
multiplied by seasonal and day of the week adjustment factor to get
an estimated AADT.
Norman W. Garrick
Characterizing Traffic Counts
Vehicles/hr or AADT
Often reports from state level traffic count studies give average annual
daily traffic (AADT)
Since hourly volume for the design hour is what is typically used for design
it is left up to the designer to come up with a reasonable design hourly
volume from the AADT
As a very rough guide the typical design hour volume is can be taken as
about 10% of the AADT. But this % varies significantly depending on
the temporal variation in traffic.
Norman W. Garrick