Archive for July 8th, 2008

The MPH Illusion: How We Misjudge Time Savings on the Road

Imagine there were two roads that could be fixed to improve drivers’ travel times. Imagine also they’re the same length, but have different average speeds.

Fixing the first road will see drivers’ speeds increase from 40 kph to 50 kph.

Fixing the second road, meanwhile, will see drivers’ speeds go from 80 kph to 130 kph.

Given that the construction costs would be the same to fix each road, which road would you choose to improve? Which would result in more time saved (and, hence, more social benefit)?

If you’re a regular, math-challenged sort like me, you probably picked the second option. But apart from some minor rounding differences, the amount of time saved for each is actually the same.

This example comes from a fascinating new paper, “Driving Speed Changes and Subjective Estimate of Time Savings, Accident Risks and Braking,” by Ola Svenson, head of the Risk Analysis, Social and Decision Research Unit at Stockholm University.

I’m admittedly someone who has trouble computing things like how much time an increase in speed will save — once you get me off the nice 60 mph mark (a mile a minute), things get a little fuzzy. But now I’ve found I’m apparently not alone. When Svenson asked this to a group of respondents, a majority thought the 80 to 120 kph option was better. The reason, he speculates, has to do with a sort of “proportion heuristic,” an effect that’s been found in many other contexts (the work of Daniel Bartells is instructive here); in what he calls a “ratio rule,” we’re biased by the ratios in speed changes, rather than employing the actual, more complex formula.

What this means on the road is that we may underestimate time savings of increases in lower speeds and, perhaps more importantly, overestimate the time savings we gain when we begin to accelerate from an already high speed. He also looked at estimations of crash risk and braking distance as it applies to speed — both of which are non-linear — and found similar mis-estimations. He notes: “Intuitive arguments for higher speeds may be biased.”

Astute readers will note the curious echo here of another study, the so-called MPG illusion, by Duke University researchers Richard P. Larrick and Jack B. Soll, which found that increases in MPG from 16 to 20 can save as much fuel, relatively, as going from 33 to 50 mpg.

Previous research has found that we tend to overestimate trip lengths, and we think the waits at things like traffic lights were longer than they really were. The lesson in Svenson’s new paper, I suppose, is that that a car’s instrument panel is more complicated than it appears.

Posted on Tuesday, July 8th, 2008 at 3:50 pm by: Tom Vanderbilt
Comments Off on The MPH Illusion: How We Misjudge Time Savings on the Road. Click here to leave a comment.
Traffic Tom Vanderbilt

How We Drive is the companion blog to Tom Vanderbilt’s New York Times bestselling book, Traffic: Why We Drive the Way We Do (and What It Says About Us), published by Alfred A. Knopf in the U.S. and Canada, Penguin in the U.K, and in languages other than English by a number of other fine publishers worldwide.

Please send tips, news, research papers, links, photos (bad road signs, outrageous bumper stickers, spectacularly awful acts of driving or parking or anything traffic-related), or ideas for my Transport column to me at:

For publicity inquiries, please contact Kate Runde at Vintage:

For editorial inquiries, please contact Zoe Pagnamenta at The Zoe Pagnamenta Agency:

For speaking engagement inquiries, please contact
Kim Thornton at the Random House Speakers Bureau:

Order Traffic from:

Amazon | B&N | Borders
Random House | Powell’s

U.S. Paperback UK Paperback
Traffic UK
Drive-on-the-left types can order the book from

For UK publicity enquiries please contact Rosie Glaisher at Penguin.

Upcoming Talks

April 9, 2008.
California Office of Traffic Safety Summit
San Francisco, CA.

May 19, 2009
University of Minnesota Center for Transportation Studies
Bloomington, MN

June 23, 2009
Driving Assessment 2009
Big Sky, Montana

June 26, 2009
PRI World Congress
Rotterdam, The Netherlands

June 27, 2009
Day of Architecture
Utrecht, The Netherlands

July 13, 2009
Association of Transportation Safety Information Professionals (ATSIP)
Phoenix, AZ.

August 12-14
Texas Department of Transportation “Save a Life Summit”
San Antonio, Texas

September 2, 2009
Governors Highway Safety Association Annual Meeting
Savannah, Georgia

September 11, 2009
Oregon Transportation Summit
Portland, Oregon

October 8
Honda R&D Americas
Raymond, Ohio

October 10-11
INFORMS Roundtable
San Diego, CA

October 21, 2009
California State University-San Bernardino, Leonard Transportation Center
San Bernardino, CA

November 5
Southern New England Planning Association Planning Conference
Uncasville, Connecticut

January 6
Texas Transportation Forum
Austin, TX

January 19
Yale University
(with Donald Shoup; details to come)

Monday, February 22
Yale University School of Architecture
Eero Saarinen Lecture

Friday, March 19
University of Delaware
Delaware Center for Transportation

April 5-7
University of Utah
Salt Lake City
McMurrin Lectureship

April 19
International Bridge, Tunnel and Turnpike Association (Organization Management Workshop)
Austin, Texas

Monday, April 26
Edmonton Traffic Safety Conference
Edmonton, Canada

Monday, June 7
Canadian Association of Road Safety Professionals
Niagara Falls, Ontario

Wednesday, July 6
Fondo de Prevención Vial
Bogotá, Colombia

Tuesday, August 31
Royal Automobile Club
Perth, Australia

Wednesday, September 1
Australasian Road Safety Conference
Canberra, Australia

Wednesday, September 22

Wisconsin Department of Transportation’s
Traffic Incident Management Enhancement Program
Statewide Conference
Wisconsin Dells, WI

Wednesday, October 20
Rutgers University
Center for Advanced Infrastructure and Transportation
Piscataway, NJ

Tuesday, March 8, 2011
Ontario Injury Prevention Resource Centre
Injury Prevention Forum

Monday, May 2
Idaho Public Driver Education Conference
Boise, Idaho

Tuesday, June 2, 2011
California Association of Cities
Costa Mesa, California

Sunday, August 21, 2011
American Association of Motor Vehicle Administrators
Milwaukee, Wisconsin

Wednesday, October 26, 2011
Attitudes: Iniciativa Social de Audi
Madrid, Spain

April 16, 2012
Institute for Sensible Transport Seminar
Gardens Theatre, QUT
Brisbane, Australia

April 17, 2012
Institute for Sensible Transport Seminar
Centennial Plaza, Sydney
Sydney, Australia

April 19, 2012
Institute for Sensible Transport Seminar
Melbourne Town Hall
Melbourne, Australia

January 30, 2013
University of Minnesota City Engineers Association Meeting
Minneapolis, MN

January 31, 2013
Metropolis and Mobile Life
School of Architecture, University of Toronto

February 22, 2013
ISL Engineering
Edmonton, Canada

March 1, 2013
Australian Road Summit
Melbourne, Australia

May 8, 2013
New York State Association of
Transportation Engineers
Rochester, NY

August 18, 2013 “Ingenuity” Conference
San Francisco, CA

September 26, 2013
TransComm 2013
(Meeting of American Association
of State Highway and Transportation
Officials’ Subcommittee on Transportation
Grand Rapids MI



July 2008

No, you probably won be compensated one million dollars; however, with the right blend of negotiating skills and patience, your efforts will be substantially rewarded!I have seen up to forty thousand dollars added to starting compensation through diligent negotiations. It is a way to significantly raise your standard of living and sense of self, simply by