The Solomon curve is a graphical representation of the collision rate of automobiles as a function of their speed compared to the average vehicle speed on the same road. The curve was based on research conducted by David Solomon in the late 1950s and published in 1964. [1] Subsequent research suggests significant biases in the Solomon study, which may cast doubt on its findings. [2]
In 1964, Solomon researched the relationship between average speed and the collision rates of automobiles and plotted the results. [3] While others have attempted to quantify the relationship between average speed and collision rates, Solomon's work was both "the earliest and best known". [4] Solomon conducted a comprehensive study of more than 10,000 collision-involved drivers and their vehicles and how other roadway, driver, and vehicle characteristics affect the probability of being involved in a crash. [5] 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. [6] Although typically called the Solomon curve, the U-shaped curve has also been referred to as the Crash Risk Curve. [7]
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. [8] The Cirillo data produced a U-shaped curve similar to the Solomon curve. [9] 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. [10] Reporting on these results in 1971, academics West and Dunn confirmed the findings of Solomon and Cirillo, [11] 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. [12]
In 1991, Fildes, Rumbold, and Leening collected self-reported crash data from 707 motorists in Australia with fewer than 200 reporting that they had been in a collision but, unlike Solomon and Cirillo, the researchers found no relationship between slower speeds and increased crash involvement. [13] 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". [14]
In July 2001, Kloeden CN, Ponte G and McLean AJ of the Road Accident Research Unit, Adelaide University quantified the relationship, "... between free travelling speed and the risk of involvement in a casualty crash in 80 km/h or greater speed limit zones in rural South Australia" using a case control study design. They found, "..the risk of involvement in a casualty crash increased more than exponentially with increasing free travelling speed above the mean traffic speed and that travelling speeds below the mean traffic speed were associated with a lower risk of being involved in a casualty crash." Outlining past research in this area, they suggest that, in the Solomon research, "Both the numerator (number of crashes in a particular speed band) and the denominator (number of vehicle-miles travelled in that same speed band) may have been quite inaccurate for relatively low speeds." [15]
Hauer provided a theoretical foundation for the Solomon curve in 1971—“for example, if I drive at 45 mph, while the median 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. [16]
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. [7] 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 Driver that control of absolute driver speed, and not variability, should be the primary focus of traffic safety regulation. [17] 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. [18] Consequently, many states and safety organizations advise drivers to “drive with the flow of traffic”. [14]
Speed limits on road traffic, as used in most countries, set the legal maximum speed at which vehicles may travel on a given stretch of road. Speed limits are generally indicated on a traffic sign reflecting the maximum permitted speed, expressed as kilometres per hour (km/h) or miles per hour (mph) or both. Speed limits are commonly set by the legislative bodies of national or provincial governments and enforced by national or regional police and judicial authorities. Speed limits may also be variable, or in some places nonexistent, such as on most of the Autobahnen in Germany.
Automotive safety is the study and practice of automotive design, construction, equipment and regulation to minimize the occurrence and consequences of traffic collisions involving motor vehicles. Road traffic safety more broadly includes roadway design.
Road traffic safety refers to the methods and measures used to prevent road users from being killed or seriously injured. Typical road users include pedestrians, cyclists, motorists, vehicle passengers, and passengers of on-road public transport.
A shoulder, hard shoulder (British) or breakdown lane is an emergency stopping lane by the verge of a road or motorway, on the right side in countries which drive on the right, and on the left side in countries which drive on the left. Many wider (U.S.) freeways, or expressways elsewhere have shoulders on both sides of each directional carriageway — in the median, as well as at the outer edges of the road, for additional safety. Shoulders are not intended for use by through traffic, although there are exceptions.
In road transport, a lane is part of a roadway that is designated to be used by a single line of vehicles to control and guide drivers and reduce traffic conflicts. Most public roads (highways) have at least two lanes, one for traffic in each direction, separated by lane markings. On multilane roadways and busier two-lane roads, lanes are designated with road surface markings. Major highways often have two multi-lane roadways separated by a median.
Rumble strips are a road safety feature to alert inattentive drivers of potential danger, by causing a tactile vibration and audible rumbling transmitted through the wheels into the vehicle interior. A rumble strip is applied along the direction of travel following an edgeline or centerline, to alert drivers when they drift from their lane. Rumble strips may also be installed in a series across the direction of travel, to warn drivers of a stop or slowdown ahead, or of an approaching danger spot.
A rollover or overturn is a type of vehicle crash in which a vehicle tips over onto its side or roof. Rollovers have a higher fatality rate than other types of vehicle collisions.
Motorcycle safety is the study of the risks and dangers of motorcycling, and the approaches to mitigate that risk, focusing on motorcycle design, road design and traffic rules, rider training, and the cultural attitudes of motorcyclists and other road users.
In transportation engineering, traffic flow is the study of interactions between travellers and infrastructure, with the aim of understanding and developing an optimal transport network with efficient movement of traffic and minimal traffic congestion problems.
Bicycle safety is the use of road traffic safety practices to reduce risk associated with cycling. Risk can be defined as the number of incidents occurring for a given amount of cycling. Some of this subject matter is hotly debated: for example, which types of cycling environment or cycling infrastructure is safest for cyclists. The merits of obeying the traffic laws and using bicycle lighting at night are less controversial. Wearing a bicycle helmet may reduce the chance of head injury in the event of a crash.
In the United States, speed limits are set by each state or territory. States have also allowed counties and municipalities to enact typically lower limits. Highway speed limits can range from an urban low of 25 mph (40 km/h) to a rural high of 85 mph (137 km/h). Speed limits are typically posted in increments of five miles per hour (8 km/h). Some states have lower limits for trucks, some also have night and/or minimum speed limits.
Transportation safety in the United States encompasses safety of transportation in the United States, including automobile crashes, airplane crashes, rail crashes, and other mass transit incidents, although the most fatalities are generated by road incidents annually killing 32,479 people in 2011 to over 42,000 people in 2022. The number of deaths per passenger-mile on commercial airlines in the United States between 2000 and 2010 was about 0.2 deaths per 10 billion passenger-miles. For driving, the rate was 150 per 10 billion vehicle-miles: 750 times higher per mile than for flying in a commercial airplane.
The National Maximum Speed Limit (NMSL) was a provision of the federal government of the United States 1974 Emergency Highway Energy Conservation Act that effectively prohibited speed limits higher than 55 miles per hour (89 km/h). It was drafted in response to oil price spikes and supply disruptions during the 1973 oil crisis and remained the law until 1995.
86 percent of people in the United States use private automobiles as their primary form of transportation to their workplace.
Road traffic collisions generally fall into one of five common types:
A traffic collision, also known as a motor vehicle collision, occurs when a vehicle collides with another vehicle, pedestrian, animal, road debris, or other moving or stationary obstruction, such as a tree, pole or building. Traffic collisions often result in injury, disability, death, and property damage as well as financial costs to both society and the individuals involved. Road transport is the most dangerous situation people deal with on a daily basis, but casualty figures from such incidents attract less media attention than other, less frequent types of tragedy. The commonly used term car accident is increasingly falling out of favor with many government departments and organizations, with the Associated Press style guide recommending caution before using the term. Some collisions are intentional vehicle-ramming attacks, staged crashes, vehicular homicide or vehicular suicide.
A roadway departure is a type of incident that occurs when a vehicle leaves the roadway. Such incidents can lead to a single-vehicle collision.
The geometric design of roads is the branch of highway engineering concerned with the positioning of the physical elements of the roadway according to standards and constraints. The basic objectives in geometric design are to optimize efficiency and safety while minimizing cost and environmental damage. Geometric design also affects an emerging fifth objective called "livability," which is defined as designing roads to foster broader community goals, including providing access to employment, schools, businesses and residences, accommodate a range of travel modes such as walking, bicycling, transit, and automobiles, and minimizing fuel use, emissions and environmental damage.
Speed limits are enforced on most public roadways by authorities, with the purpose to improve driver compliance with speed limits. Methods used include roadside speed traps set up and operated by the police and automated roadside 'speed camera' systems, which may incorporate the use of an automatic number plate recognition system. Traditionally, police officers used stopwatches to measure the time taken for a vehicle to cover a known distance. More recently, radar guns and automated in-vehicle systems have come into use.
In legal terminology, the assured clear distance ahead (ACDA) is the distance ahead of any terrestrial locomotive device such as a land vehicle, typically an automobile, or watercraft, within which they should be able to bring the device to a halt. It is one of the most fundamental principles governing ordinary care and the duty of care for all methods of conveyance, and is frequently used to determine if a driver is in proper control and is a nearly universally implicit consideration in vehicular accident liability. The rule is a precautionary trivial burden required to avert the great probable gravity of precious life loss and momentous damage. Satisfying the ACDA rule is necessary but not sufficient to comply with the more generalized basic speed law, and accordingly, it may be used as both a layman's criterion and judicial test for courts to use in determining if a particular speed is negligent, but not to prove it is safe. As a spatial standard of care, it also serves as required explicit and fair notice of prohibited conduct so unsafe speed laws are not void for vagueness. The concept has transcended into accident reconstruction and engineering.
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