Solomon curve
Encyclopedia
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The Solomon curve is the graphical representation of the research
conducted by David Solomon in the late 1950s and published in 1964.
speed
and collision
rates of automobile
s and plotted the results. While others have attempted to quantify the relationship between average speed and collision rates, Solomon's work was both "the earliest and best known". Solomon conducted a comprehensive study of more than 10,000 accident-involved
drivers and their vehicles and how other roadway, driver, and vehicle characteristics affect the probability of being involved in a crash. He found that the probability of being involved in a crash per vehicle-mile as a function of on-road vehicle speeds follows a U-shaped curve with speed values around the median speed having the lowest probability of being in a crash. Although typically called the Solomon curve, the U-shaped curve has also been referred to as the Crash Risk Curve.
of the pack is 60 mph, how many cars will pass me in an hour and hence have a chance to collide with me ” – that showed that the theoretical distribution
was nearly identical to the Solomon curve.
that control of absolute driver speed, and not variability, should be the primary focus of traffic safety regulation. Both views support the fact that the seminal research underlying the Solomon curve shows that the greater the difference between a driver’s speed and the average speed of traffic – both above and below that average speed – the greater the likelihood of involvement in a crash. Consequently, many states and safety organizations advise drivers to “drive with the flow of traffic”.
The Solomon curve is the graphical representation of the research
Research
Research can be defined as the scientific search for knowledge, or as any systematic investigation, to establish novel facts, solve new or existing problems, prove new ideas, or develop new theories, usually using a scientific method...
conducted by David Solomon in the late 1950s and published in 1964.
The original research
In 1964, Solomon researched the relationship between averageAverage
In mathematics, an average, or central tendency of a data set is a measure of the "middle" value of the data set. Average is one form of central tendency. Not all central tendencies should be considered definitions of average....
speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
and collision
Collision
A collision is an isolated event which two or more moving bodies exert forces on each other for a relatively short time.Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies...
rates of automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
s and plotted the results. While others have attempted to quantify the relationship between average speed and collision rates, Solomon's work was both "the earliest and best known". Solomon conducted a comprehensive study of more than 10,000 accident-involved
drivers and their vehicles and how other roadway, driver, and vehicle characteristics affect the probability of being involved in a crash. He found that the probability of being involved in a crash per vehicle-mile as a function of on-road vehicle speeds follows a U-shaped curve with speed values around the median speed having the lowest probability of being in a crash. Although typically called the Solomon curve, the U-shaped curve has also been referred to as the Crash Risk Curve.
Subsequent research
In 1968, Julie Cirillo conducted a similar study of 2,000 vehicles on interstate highways that addressed speed variation’s impact on crashes that involved two or more vehicles. The Cirillo data represented a U-shaped curve similar to the Solomon curve. Two potential problems with the Solomon and Cirillo studies were (1) the police reports, driver’s reports, and third party estimates used were subject to error and unknown reliability, and (2) many of the crashes involving slow speed likely involved vehicles that were stopping or slowing to turn or just entering the road. The Research Triangle Institute conducted a study in 1970 where data was collected on 114 crashes involving 216 vehicles on a state highway in Indiana to address these concerns by (1) combining automated, embedded speed monitoring stations with trained on-scene crash investigators, and (2) distinguishing data on vehicles slowing to negotiate a turn from vehicles moving slowly in the flow of traffic. Reporting on these results in 1971, academics West and Dunn confirmed the findings of Solomon and Cirillo, but found that crashes involving turning vehicles accounted for 44 percent of all crashes observed in the study and that excluding these crashes from the analysis greatly attenuated the factors that created the U-shape of the Solomon curve. In 1991, Fildes, Rumbold, and Leening collected self-reported crash data from 707 motorists in Australia with less than 200 reporting they had been in an accident but, unlike Solomon and Cirillo, the researchers found no relationship between slower speeds and increased crash involvement. Notwithstanding the many studies over the years, in testimony before the Ohio Senate Highways and Transportation Committee on June 10, 2003, Julie Cirillo, Former Assistant Administrator and Chief Safety Officer for the Federal Motor Carrier Safety Administration (FMCSA), testified that "up to the present time there has been no evidence to alter Solomon’s original finding that variance from the mean operating speed is a major contributor to accidents."Theoretical foundation
Hauer provided a theoretical foundation for the Solomon curve in 1971 – “for example, if I drive at 45 mph, while the medianMedian
In probability theory and statistics, a median is described as the numerical value separating the higher half of a sample, a population, or a probability distribution, from the lower half. The median of a finite list of numbers can be found by arranging all the observations from lowest value to...
of the pack is 60 mph, how many cars will pass me in an hour and hence have a chance to collide with me ” – that showed that the theoretical distribution
Frequency distribution
In statistics, a frequency distribution is an arrangement of the values that one or more variables take in a sample. Each entry in the table contains the frequency or count of the occurrences of values within a particular group or interval, and in this way, the table summarizes the distribution of...
was nearly identical to the Solomon curve.
Practical implications
While Solomon’s research focused on speed deviation, and not speed per se, some commentators have cited Solomon’s research to support the conclusion that raw speed and crash risk are not directly related. On the other hand, while recognizing Solomon's research reveals the importance to safety of variability around average speed, Leonard Evans concludes in Traffic Safety and the DriverTraffic Safety and the Driver
Traffic Safety and the Driver, ISBN 0-442-00163-0, is a book authored by Leonard Evans, published in 1991 by Van Nostrand Reinhold .-Chapter headings:1. Introduction 2. Effects of Sex and Age 3. An Overview of US Traffic Fatalities...
that control of absolute driver speed, and not variability, should be the primary focus of traffic safety regulation. Both views support the fact that the seminal research underlying the Solomon curve shows that the greater the difference between a driver’s speed and the average speed of traffic – both above and below that average speed – the greater the likelihood of involvement in a crash. Consequently, many states and safety organizations advise drivers to “drive with the flow of traffic”.
See also
- Automobile safetyAutomobile safetyAutomobile safety is the study and practice of vehicle design, construction, and equipment to minimize the occurrence and consequences of automobile accidents. Automobile safety is the study and practice of vehicle design, construction, and equipment to minimize the occurrence and consequences of...
- Car accidentCar accidentA traffic collision, also known as a traffic accident, motor vehicle collision, motor vehicle accident, car accident, automobile accident, Road Traffic Collision or car crash, occurs when a vehicle collides with another vehicle, pedestrian, animal, road debris, or other stationary obstruction,...
- Design speedDesign speedThe design speed is a tool used to determine geometric features of a new road during road design. The design speed chosen for a highway is a major factor in choosing superelevation rates and radii of curves, sight distance, and the lengths of crest and sag vertical curves...
- Operating speedOperating speedThe operating speed of a road is the speed at which motor vehicles generally operate on that road.The precise definition of "operating speed", however, is open to debate. Some sources, such as the AASHTO, have changed their definitions recently to match the common use of the word...
- Road-traffic safetyRoad-traffic safetyThe term road traffic safety is about the risk of a person being killed or seriously injured while using the road network as a pedestrian, cyclist, motorist or user of on road public transport...
- Speed limitSpeed limitRoad speed limits are used in most countries to regulate the speed of road vehicles. Speed limits may define maximum , minimum or no speed limit and are normally indicated using a traffic sign...