[sustran] Litman Generated Traffic Paper

[Paul Barter]

Dear sustran-discussers,
This is from the alt-transp list.  Todd Litman is not on sustran-discuss so
if you want to give him comments you will need to email him directly.
Best wishes and happy new year,
Paul.


From: litman at IslandNet.com (Todd Litman)
Date: Tue, 30 Dec 1997 17:42:06 -0800 (PST)
Subject: alt-transp Generated Traffic Paper

Seasons greetings from Victoria, BC.

I have been invited to present a paper on incorporating generated traffic
into transportation decision making to World Bank staff next month, while I
am in Washington DC for the Transportation Research Board annual meeting.
The introductory section of the paper is copied below (excluding citations,
which don't copy into an email format).

I would appreciate getting feedback on this draft. Please let me know if you
would like me to email you the full paper.

Sincerely,

Todd Litman, Director
Victoria Transport Policy Institute
"Efficiency - Equity - Clarity"
1250 Rudlin Street
Victoria, BC, V8V 3R7, Canada
Phone & Fax: (250) 360-1560
E-mail:     litman at islandnet.com
Website:    http://www.islandnet.com/~litman
===================================================================

                      Generated Traffic
        Implications for Transport Planning

                Presentation to World Bank
                    13 January 1997
                            by
                       Todd Litman
              Victoria Transport Policy Institute


Introduction
Traffic engineers often compare traffic flow to a fluid, but in congested
situations it is better described as a gas, which expands to fill available
space. Transport improvements that reduce user costs (including vehicle
operating costs and travel time) tend to attract trips from other routes,
times and modes, and increase total travel. This is called generated traffic
or induced travel.  Economic analysis requires that all incremental (net)
benefits and costs be including in the evaluation of policies, programs and
projects. Incremental costs and benefits to existing trips are relatively
easy to determine, but special consideration is needed to determine net
benefits and costs of generated traffic.

Generated traffic has three implications for transportation decision making.

1. First, generated traffic provides relatively less benefits than existing
trips, since it consists of lower value trips that consumers are most
willing to forego due to congestion delays.

2. Second, generated traffic erodes much of the predicted congestion
reduction.  Studies have found that 40-90% of added highway capacity is
filled with generated traffic,  and in some cases congestion costs actually
increase. One researcher concludes,

"At the country level we find that a 1.0 percent increase in lane-miles soon
induces an immediate 0.2 percent increase in traffic, building to a 0.6
percent increase within two years after the lane-miles are added. At the
metropolitan level, the immediate effect is also about 0.2 percent, building
to a 0.9 percent increase after four years. Therefore, it appears that
adding road capacity does little to decrease congestion because of the
substantial induced traffic."

3. The third implication is that generated travel increases external costs
such as accidents, congestion on other roads (for example, increasing
freeway capacity oiljkincreases traffic volumes and therefore congestion on
surface streets), parking demand (and parking subsidies), noise, air
pollution and energy consumption.  These external costs are significant,
particularly under urban peak-period conditions.

While users' marginal benefits exceed their marginal costs (if not, users
would not take the additional trips), these benefits do not necessarily
exceed total incremental costs. Generated traffic, therefore, may create
more costs than benefits.


Failing to incorporate generated traffic into transport modeling and
planning, underestimating generated traffic, or ignoring any of these three
factors can significantly affect transport decision making. A report by the
UK Standing Advisory Committee on Trunk Road Assessment concludes, "... the
economic value of a [road] scheme can be overestimated by the omission of
even a small amount of induced traffic. We consider this matter of profound
importance to the value-for-money assessment of the road programme."

Models that fail to consider generated traffic were found by one study to
overvalue roadway capacity expansion benefits by 50% or more.  One
researcher states,
"...at moderate levels of congestion and elasticity values, the fixed matrix
measure of benefits [which ignores generated traffic] exceeds the variable
demand measure [which incorporates generated traffic] by 20-50 per cent. An
important finding is that quite small absolute changes in traffic volumes
have a significant impact on the benefit measures. Of course, the
proportional effect on scheme Net Present Value will be greater still."

Failing to incorporate feedback (congestion impacts on future travel) into
traffic modeling can significantly overpredict traffic congestion problems.
One study found that incorporating feedback when modeling a congested road
network increased predicted traffic speeds by more than 20%, and reduced
predicted VMT by more than 10%.  Another study found that the ranking of
preferred projects changed significantly when generated traffic feedback is
incorporated into project assessment analysis.  Specifically, capacity
expansion options provide less congestion reduction benefit and increase air
emissions, while demand management and No Build options offer greater benefits.

Traffic models used in some large urban areas incorporate generated traffic
feedback,  but traffic models used in medium and small communities usually
do not, and generated traffic costs are seldom incorporated in the economic
analysis of specific projects.  This omission is often justified with the
excuse that tools do not exist to predict how much traffic will be generated
or resulting net costs. These excuses are no longer valid.