Some Dim

Over at Freakonomics, Steven Dubner wonders about the ethics of dimming headlights in the face of oncoming traffic (e.g., why do drivers perform this everyday altruism in the face of seemingly small consequences for not doing so). He also asks: “What I’d like to know is whether the benefit of dimming your headlights — that is, the benefit of not blinding the oncoming driver — is indeed larger than the benefit of keeping your own brights burning?”

This is a question that people who study vision and lighting and driving have thought about a lot. To summarize a conversation I had with Michael Sivak, at UM-TRI, there’s three distances involved here: The legally required distance to dim one’s lights in the face of oncoming vehicles, the optimal distance for maintaining one’s own visibility (and, I suppose, not blinding the other), and then what drivers’ actually do. Readers of this blog will suspect the last factor does not often match up with the two previous factors (and, I should add, as with many things in driving, the scientific issues around night-time illumination are much more complex than the “average expert driver” — i.e., everyone — realizes).

A paper by Kare Rumar expounds on this question:

From a pure visibility point of view, opposing drivers should never dim their lights, but should drive on high beam through the whole meeting process. There are, however, certainly other reasons for dimming the lights, such as discomfort glare and fatigue over a longer period with repeated high-beam meetings.

The study of Helmers and Rumar (1975) indicates that the improvements in the low beam since the fifties and sixties have been considerable. That is probably the main reason why the high-beam visibility curve and the low-beam visibility curve in later studies do cross each other—at least when the intensity differences between the two opposing high beams are not too large (about triple or less).

When the two opposing high beams differ substantially in intensity, the visibility differences between the two opposing drivers are quite pronounced (see Figure 3). In such situations, it is most probable that the driver with the weaker high beams will be the one who wants to initiate the dimming, because the driver with the weaker high beam experiences substantial disability and discomfort glare. On a straight, flat road, such a driver will want to dim the high beams at a very large distance between the vehicles.

An early dimming means that both drivers will have to drive on low beams for an extensive part of the meeting process. However, as stated above and illustrated in Figures 2 and 3, at larger separations low beams normally offer shorter visibility distances than high beams. This means that an early dimming leads to short visibility distances for a greater distance traveled, for both opposing drivers.

This entry was posted on Tuesday, September 28th, 2010 at 12:25 pm and is filed under Traffic safety, Traffic Wonkery. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

11 Responses to “Some Dim”

  1. Richard Masoner Says:

    Broken link to Freakanomics — looks like you missed initial “H” for http on the cut and paste.

  2. John Says:

    It is a courtesy– what should be at the heart of much of driving while in any traffic. You should not blind other drivers.

  3. Thomas Says:

    What about the likely consequences of blinding the other driver and causing an accident. If they drive off the road, the driver that failed to dim their lights could be held responsible. If they cross the center line and hit the vehicle with the brights on, the driver that failed to dim can suffer property damage, injury or death.

    It seems like a pretty obvious case of self-interest at work to me.

  4. Alex Turini Says:

    The above is a link to a collision report between an overturned semi-trailer and a freight train, rural Australia, 2006.

    One of the factors of the collision is that the freight train engineers had “dimmed” their locomotive’s headlights shortly before the collision due to a motor vehicle passing on an adjacent road.

    The detailed report outlines a raft of minor, preventable errors that occured prior to the collision, however the official category the Austrlaian Transport Safety Bureau placed this incident in was “accident”.

  5. alvin Says:

    Instead of declaring that the benefits are opposing, i.e. whether the benefit of not blinding the oncoming driver is indeed larger than the benefit of keeping one’s own brights burning, I see it a little differently, and maybe Kare Rumar hints at it also.

    Kare Rumar’s paper states: “drivers very often dim too early from the visibility point of view. Research clearly shows that visibility for both opposing drivers would improve if they dimmed their high beams at shorter distances. These findings underscore the fact that the fear of late dimming, as illustrated in the dimming legislations in all U.S. states, is exaggerated.”

    I find that, in a two-opposing-cars situation, the driver who dims first encourages the second driver to reciprocate. When I drive, I dim my lights earlier than strictly necessary to remind the oncoming driver to do the same; by the time the other driver gets the hint and dims, hopeably the separation between cars is still ample enough to avoid being blinded.

    So maybe the combined benefit of not blinding the oncoming driver AND improving the likelihood of not being blinded in return is indeed larger than the benefit of keeping your own brights burning. Besides, as a loss-averse person, I value not being blind much more highly than I value driving with brights instead of dims.

    Next, I’d like to see if people dim less quickly/often when faced with oncoming lowbeams instead of oncoming highbeams, and also if drivers who sit high in tall vehicles are less likely to notice blinding effects and therefore are less likely to dim their own lights.

  6. alvin Says:

    I recall that some states’ driving manuals have a dual strategy for minimizing high-beam blindness on their roadways.
    Here’s New Jersey’s:
    Bright beams can momentarily blind other motorists […] and should not be used if other vehicles are approaching or when driving behind another vehicle. […] At a speed of 50 mph, a motorist will have traveled the length of a football field while being unable to see. If a vehicle is approaching with high beams, a motorist should look to the right of the road until the vehicle passes.

    I myself was specifically instructed to look at the reflective paint line marking the right edge of the lane as a guide. I imagined that this rule fostered in fellow drivers a “don’t be like those jerks who might blind you” ethos, i.e. that the strategy for coping with the improper brights of others is a reminder to not use improper brights oneself.

  7. Theophylact Says:

    I’m regularly blinded by someone following me with high beams on, especially when it’s an SUV with high-mounted headlights. The glare in my rear-view mirror can black out the road in front almost completely.

  8. Kevin Love Says:

    There is very little altruism around here. Those who don’t dim get the high beams turned back on at them and get blasted with the high beams until they dim.

  9. John Says:

    And you know what’s funny? Altriusm does indeed work here– its flip side is induced guilt. If I have a driver approaching me with anything resembling high beams– a big SUV, misaimed lights, true high beam forgetfulness, other… I flash my highs at them, usually twice rapidly. Sometimes the other driver complies, turns to low beams. Numerous times, they do not change them. I then go to what is written above– you look at the right side line. I never get blinded, but more importantly, what you have done with your flashing the high beams is let the other driver know that they, essentially, are wrong. Make no mistake– driving is high-stakes emotional pride, and the other driver will feel guilt. Their behavior will change somehow down the road.

    The other thing that might be mentioned here is how fast a car will turn on its high beams after passing you, or you them. I like to play this “quickdraw game,” and I have never been outdrawn yet.

    Number one Google hit for “best driver in the world”

  10. gregorylent Says:

    never done in india .. in fact, the opposite happens, just in case the beams were accidentally on low .. hate it, but it ain’t my country

  11. DoctorJay Says:

    Funny, I find it rare that I need to turn on my high beams. There’s usually so many other lights that I don’t really use them. Maybe on a 2 lane, winding country road, but otherwise the streetlights and other cars provide plenty of illumination.

    The writer on Freakonomics talks of using high beams to drive in the rain. That’s a great way to bounce the light right back into your own face.

Leave a Reply

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



September 2010
« Aug   Oct »

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