Critiquing the "Urban Mobility Report"
http://www.planetizen.com/node/61087
By Todd Litman, March 6, 2013
One of a planner’s main jobs is to produce objective technical
analysis. We assemble and organize data so the facts can speak for
themselves. However, behind most technical reports is a dramatic story.
For example, the
Urban Mobility Report
(UMR) calculates traffic congestion costs for U.S. cities. Behind the
numbers is the following narrative: Automobile commuters are good,
hard-working people victimized by villainous traffic congestion, which
threatens our productivity and happiness. Traffic engineers are heroes
who can defeat this monster if only they are given sufficient resources.
Success requires that wise but distant decision-makers (city
councilors, transportation agencies, governors, congress, etc.) be
persuaded to support their congestion reduction campaign. With more
highway funding we will once again enjoy growth and happiness.
What’s wrong with this narrative? A lot if you care about transport
system efficiency and equity. It assumes that “transportation” means
automobile travel, ignoring other modes. It assumes that congestion is
the most important transport planning issue, ignoring other objectives
and impacts. It ignores other factors that affect mobility and
accessibility, and the indirect costs of highway expansion. It ignores
motorists’ complicity in creating congestion when they choose
automobile-dependent lifestyles, and as citizens when they oppose
efforts to improve alternative mode and transport pricing reforms. This
narrative directs resources to roadway expansion over more efficient and
equitable solutions.
The
Urban Mobility Report is
biased in various ways that exaggerate congestion costs and roadway
expansion benefits. It ignores basic research principles: it provides no
literature review, it fails to explain key assumptions, it inadequately
cites sources, and incorporates no external peer review. Yet, few
practitioners, decision-makers or journalists who use UMR results seem
aware of these problems.
My new report
Putting Congestion in its Place: Critical Evaluation of the “Urban Mobility Report” investigates these issues. This blog describes my key findings. For more detailed information please read the full report.
Defining Congestion
The
Urban Mobility Report’s title is inaccurate: it should be renamed the
Traffic Congestion Report since it ignores other modes and performance factors. The UMR uses the terms
commuter when the analysis only considers
automobile commuters. This significantly skews results. For example, the UMR indicates that in 2011 Washington DC’s
automobile commuters experienced
67 average annual hours of delay, but since that region has only 43%
auto commute mode share this averages just 29 hours per
commuter. In contrast, Houston’s
automobile commuters experience a somewhat lower 52 annual hours of delay, but since it has a 88% auto mode share this averages 46 hours per
commuter
overall, much higher than Washington DC. This shows how alternative
modes can help reduce total congestion costs. Yet, the UMR only values
walking, cycling and public transit to the degree they reduce vehicle
congestion; it assigns no benefit to the congestion avoided by the
travelers who shift mode.
Congestion Costing Methods
Although not mentioned in the
Urban Mobility Report,
there is considerable debate among transport economists concerning how best to measure congestion costs. One key issue is the
baseline (or
threshold)
speed below which congestion delays are calculated. Lower baselines
result in lower congestion cost values. The UMR uses freeflow speeds
(level-of-service
A) as a baseline. For example, if a
roadway’s legal speed limit is 60 mph, and congestion reduces
peak-period traffic speeds to 50 mph, the UMR considers the 10 mph speed
reduction wasted time. However, freeflow speeds are not optimal: 45-55
mph traffic speeds tend to maximize roadway capacity and vehicle fuel
efficiency. As a result, many transport economists recommend using lower
baseline speeds.
Another key factor is the dollar value assigned to delay. The
U.S. Department of Transportation
recommends valuing personal travel at 35% to 60% of prevailing incomes,
or $8.37 to $14.34 per hour, based on $23.90 overall national average
incomes. The UMR uses $16.79 per hour (although it cites $8 per hour on
page 24 and $16 on pages 25-31), 40% more than the USDOT’s $12 per hour
default value.
The
Urban Mobility Report's congestion cost estimates
therefore represent upper-bound values while more reasonable assumptions
would result in much lower congestion cost estimates. The following
graph compares the UMR's $121 billion upper-bound cost estimate based on
a freeflow speed baseline and $16.79 per hour time costs with a
middle-range value based on 70% baseline and $12 per hour value, and a
lower-range value based on a 50% baseline and $8.37 per hour.
Congestion Cost Ranges
The Urban Mobility Report uses an upper-bound travel speed
baseline and travel time unit costs. Most economists recommend lower
values. The lower-bound estimate is based on Transport Canada’s lower
baseline speed and the U.S. Department of Transportation’s lower travel
time unit costs.
The
Urban Mobility Report uses an upper-bound travel speed
baseline and travel time unit costs. Most economists recommend lower
values. The lower-bound estimate is based on Transport Canada’s lower
baseline speed and the U.S. Department of Transportation’s lower travel
time unit costs. T
he UMR does not explain how it selected its assumptions or discuss possible analysis biases, which violates basic research principles.
Comparing Costs
The
Urban Mobility Report claims that traffic congestion
wastes “massive” amounts of time and money, estimated at 5.5 billion
hours and 2.9 billion gallons of fuel, worth an estimated $121 billion.
Described this way the costs do seem large, but measured per capita they
seem modest: 17 hours, 9 gallons and $388 per year, or less than three
minutes, 0.03 gallons and $1.06 per day. These represent less than 2% of
total travel time and fuel costs.
The following graph compares various transportation costs, measured
annually per capita. Congestion costs are estimated to range from $110
(50% baseline speeds and $8.37 per hour time costs) up to $388 (the
UMR’s estimate) compared with approximately $2,600 in vehicle ownership
costs, $1,500 in crash damages, $1,200 in parking costs, $500 in
pollution damage costs and $325 in roadway costs.
Costs Ranked by Magnitude
Congestion costs are estimated to range between about $110 and
$340 annual per capita, depending on assumptions. These are modest
compared with other transportation costs.
Congestion costs are estimated to range between about $110 and $340
annual per capita, depending on assumptions. These are modest compared
with other transportation costs.
Because congestion costs are overall modest, it is inappropriate to
evaluate potential congestion reduction strategies in isolation: a
strategy may provide little net benefit if it increases other costs, but
is worth far more if it reduces other costs or provides other benefits.
For example, roadway expansions may seem cost effective considering
just congestion reduction benefits, but if they induce additional
vehicle travel which increases other traffic problems, the overall
economic impacts may be negative. On the other hand, improving walking,
cycling and public transit, or efficient parking pricing may seem to
provide only modest congestion reduction benefits, but turn out to be
very cost effective considering their co-benefits.
The
Urban Mobility Report incorporates a striking example of
bias hidden in its technical analysis: it assumes that any traffic
speed increase reduces per-mile fuel consumption and emission rates,
although
most other studies
find that these rates minimize at 45-55 miles per hour and increase
with higher speeds, as illustrated below. As a result, the UMR assumes
that reducing congestion always saves energy and reduces emissions
although other researchers conclude otherwise. For example,
Barth and Boriboonsomin
explain, “If moderate congestion brings average speeds down from a
free-flow speed over 70 mph to a slower speed of 45 to 55 mph, this
moderate congestion can reduce CO2 emissions. If congestion mitigation
raises average traffic speed to above about 65 miles per hour, it can
increase emissions. And, of course, speeds above 65 or 70 also make the
roadway more dangerous.”
Emission Curves Compared
The
Urban Mobility Report ignores this last point, that congestion reductions can
increase traffic risks,
although it is much discussed by traffic safety researchers. Crash
rates tend to be lowest on moderately congested roads (V/C=0.6), and
increase at lower and higher congestion levels, while fatalities decline
at high levels of congestion, indicating a tradeoff between congestion
and safety. Per capita traffic deaths tend to increase with per capita
vehicle travel, so if roadway expansions induce additional vehicle
travel this tends to increase traffic casualties.
Exaggerating Future Congestion Problems
The
Urban Mobility Report’s press release headline, “As
traffic jams worsen, commuters allow extra time for urgent trips…”
implies that congestion problems are increasing, but the analysis
actually indicates that congestion has decreased in recent years due to
demographic and economic trends that have caused
vehicle travel to peak in North America.
Yet, the UMB simply extrapolates the high pre-2006 traffic growth rates
without accounting for underlying demographic and economic factors that
affect travel demands. In other words, the UMR assumes that the future
will simply be a repeat of the past, ignoring fundamental changes in
travel demands.
Conclusions and Recommendations
The
Urban Mobility Report has significant omissions and biases.
- It fails to review current congestion costing literature,
discuss the advantages and disadvantages of different methods, and
explain why the selected methodology and assumptions were chosen.
- It uses methods and assumptions that result in upper-bound
congestion cost estimates, including freeflow travel speed baseline,
although many experts recommend lower values, and $16.79 per hour travel
time values, far higher than recommended by the USDOT.
- It fails to consider factors that significantly affect accessibility
and transport costs, including the quality of transport options,
roadway connectivity, and land use proximity, and ways that various
congestion reduction strategies can affect these factors.
- It exaggerates congestion compared with other transportation costs.
It calls congestion costs “massive,” although they are relatively modest
overall, at most increasing travel time and fuel consumption by 2%.
- It ignores generated traffic and induced travel
impacts, which reduce congestion reduction benefits and increase
indirect and external costs, reducing roadway expansion net benefits.
- It predicts that traffic congestion problems will increase
significantly in the future, based on the assumption that traffic will
grow as rapidly in the future as during the peak growth years of the
late twentieth century. This ignores widely-recognized demographic and
economic trends which are changing travel demands. In fact, most experts
predict that congestion problems are unlikely to increase significantly
in the future, and can be reduced effectively by meeting growing
demands for alternative travel options and using TDM strategies.
The
Urban Mobility Report’s estimates represent the
upper-bound range of possible congestion costs; applying methodologies
and assumptions generally recommended by economists and government
agencies can reduce these estimates by half or two-thirds. To be
comprehensive and objective, the UMR should summarize current congestion
costing research; discuss different evaluation perspectives and costing
methods; explain why the methods and assumptions it uses were selected;
apply sensitivity analysis; compare congestion with other transport
costs; account for changing travel demands; consider all impacts when
evaluating potential congestion reduction strategies, and provide more
transparency, quality control and peer review.
This is not to deny that traffic congestion is a problem: it is
frustrating, increases costs and reduces productivity. However, it is
not appropriate to exaggerate congestion relative to other related
problems or evaluate congestion reduction strategies without considering
other objectives and impacts.
For More Information
Robert L. Bertini (2005),
You Are the Traffic Jam: An Examination of Congestion Measures, TRB Annual Meeting (
www.trb.org); at
www.its.pdx.edu/pdf/congestion_trb.pdf.
Joe Cortright (2010),
Driven Apart: How Sprawl is Lengthening Our Commutes and Why Misleading Mobility Measures are Making Things Worse, CEOs for Cities (
www.ceosforcities.org); at
www.ceosforcities.org/work/driven-apart.
Eric Dumbaugh (2012),
Rethinking the Economics of Traffic Congestion, Atlantic Cities, 1 June 2012; at
www.theatlanticcities.com/commute/2012/06/defense-congestion/2118.
Susan Grant-Muller and James Laird (2007), International Literature
Review of the Costs of Road Traffic Congestion, Scottish Executive (
www.scotland.gov.uk); at
www.scotland.gov.uk/Publications/2006/11/01103351/0.
Zachary Howard and Clark Williams-Derry (2012),
How Much Do Drivers Pay For A Quicker Commute? New Evidence Suggests That It's Less Than We Think, Sightline Institute (
www.sightline.org); at (
http://daily.sightline.org/2012/08/01/how-much-do-drivers-pay-for-a-quicker-commute.
Todd Litman (2009), “Congestion Costs,”
Transportation Cost and Benefit Analysis; Techniques, Estimates and Implications, Victoria Transport Policy Institute (
www.vtpi.org/tca).
Todd Litman (2012),
Smart Congestion Relief: Comprehensive Analysis Of Traffic Congestion Costs and Congestion Reduction Benefits, paper P12-5310, Transportation Research Board Annual Meeting (
www.trb.org); at
www.vtpi.org/cong_relief.pdf.
TC (2006),
The Cost Of Urban Congestion In Canada, Transport Canada (
www.tc.gc.ca); at
www.adec-inc.ca/pdf/02-rapport/cong-canada-ang.pdf.
TTI (2012),
Urban Mobility Report, Texas Transportation Institute (
http://mobility.tamu.edu/ums/report); at
http://tti.tamu.edu/documents/mobility-report-2012.pdf.
