Brush up on the Solomon Curve

Solomon U-shaped Curve

Normally cited by those wishing slow-moving vehicles should never use any roadway, even though it is perfectly legal, is the Solomon u-shaped curve, the claim is typically something to the effect of it’s safest when all traffic travels at the same speed; ergo a vehicle must never go slower than the “speed of traffic” and must not be on the roadway in the first place. A corollary to this line of thinking is the belief that posted speed limits are really minimum speed limits.

A USDOT paper, Synthesis of Safety Research Related to Speed and Speed Management 1998 has a good backgrounder on the theoretical basis of these ideas:

In a landmark study of speed and crashes involving 10,000 drivers on 600 miles (970 kilometers) of rural highways, Solomon (1964) found a relationship between vehicle speed and crash incidence that is illustrated by a U–shaped curve. Crash rates were lowest for travel speeds near the mean speed of traffic, and increased with greater deviations above and below the mean.


Excluding these (vehicles entering or vehicles slowing to leave the roadway) crashes from the analysis greatly attenuated the factors that created the U–shaped curve characteristic of the earlier studies. Without vehicles slowing to turn, or turning across traffic, the investigators found the risk of traveling much slower than average was much less pronounced. Crash risk was greatest for vehicles traveling more than two standard deviation above the mean speed.

It’s also wasn’t clear to me how or if any of this has to do with urban or suburban streets — where by definition, traffic starts and stops for a variety of necessary reasons.

There was an article published Jan 2015 in fiverthirtyeightdiscussing these concepts as well as some background of NYC’s move to make the default city-wide speed limit 25mph, down from 30mph.

Traffic Safety and Human Behavior
There’s a updated 2017 edition of Traffic Safety and Human Behavior: Second Edition, edited by David Shinar where, of course, the Solomons’ curve issue is dealt with; from the chapter Speed and Safety:

… Solomon (1964) found that drivers who drove either significantly above or below the prevailing average traffic speed were more likely to have crashes than drivers who drove at speeds close to the average. However, most crash-involved slow drivers were turning at the time of the crash; and when turning vehicles were removed from the analysis only those driving at speeds significantly above the traffic speed remained over-invovled in crashes (Fildes and Lee, 1993)

He later goes into a finely detailed review of Solomon (rural only; data from the 1950’s, etc) as well as reviewing a plethora of research published in the decades since that attempts to correct Solomon’s short-comings.

Shintar also notes that speed, when self-reported by (presumably speeding) drivers involved in a crash tend to be below actual speeds because of “Stannard’s Law” which states “drivers tend to explain their traffic accidents by reporting circumstances of lowest culpability compatible with credibility (Aronoff, 1971)”

NTSB: Reducing Speeding-Related Crashes Involving Passenger Vehicles

In July 2017, the NTSB (National Transportation Safety Board) released a major study: SS1701 Reducing Speeding-Related Crashes Involving Passenger Vehicles. In discussing Solomon 1964, it echo’s Shintar “These studies generally provided consistent evidence that driving faster than the surrounding traffic increased crash involvement rates; the evidence was less conclusive with respect to driving slower than the surrounding traffic (Aarts and van Schagen 2006)”. The report goes on to question the concept of setting speed limits at the 85th percentile on roads that are not freeways.

Speed and Crash Risk

Speed and Crash Risk, irtad (part of OECD) issued a 2018 report on the issue echoing the same sentiments about mis-use of Solomon’s work.

National Motorist’s Association

The NMA, or those identifying with it, commonly spread propaganda based on mis-stating Solomon’s 1964 work; see e.g. links in this rebuttal The National Motorists Association’s drive toward alternative facts,

One Response to “Brush up on the Solomon Curve”
  1. Andres says:

    This comment is rather weak “It’s also wasn’t clear to me how or if any of this has to do with urban or suburban streets — where by definition, traffic starts and stops for a variety of necessary reasons.”

    The reason for a start or stop doesn’t change the crash risk of speed differentiation. You can’t yell at someone to “avoid rear-ending you” just because you are “stopping for a “valid reason.” People get crashed into regardless of a valid or invalid reason for a maneuver, and speed differentiation is a huge part of that.

    (Fildes and Lee, 1993) fails to account for the fact that many freeways and highways lack turns, or particularly left turns, so a U-shaped curve on those roadways has to be explained by something other than turns.

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