Understanding the Problem
Quite simply, speeding is dangerous, and can increase the risk of crashes and the severity of injury when a crash occurs. Research is clear that higher speed is related to increased crash and injury risk at both the individual driver level, and at a road section level (Aarts & van Schagen, 2006; Elvik et al., 2019). Small changes in average roadway speed can have a large impact on safety (Elvik et al., 2019) for all crash types. The higher the impact speed, the greater the fatality and injury risk to pedestrians and other non-motorists, and this risk starts rising dramatically at speeds above 20 to 25 mph (Tefft, 2013). Speeding contributes to these risks, and to speed variation within a traffic stream, which has also been found to increase crash risk.
Managing speed is therefore an essential aspect of a systems approach that aims to prevent the worst outcomes (death or serious injury) when other parts of the system of users, vehicles, and roadway fail to prevent a crash. Lower speeds can also help to reduce the societal and individual costs of crashes and injury and the burden on post-crash care. Accordingly, speed management is widely considered an essential pillar of a Safe Systems approach to road safety (Dumbaugh et al., 2019; FHWA, 2020).
Despite the evidence that limiting speed reduces the risk of serious and fatal injuries, managing high speeds and speeding behavior remains a consistent challenge for traffic safety professionals. Highway speed limits have been increased in many States in recent years leading to higher speeds and an increased number of crashes. (See the Lower Speed Limits countermeasure section.) Speeding has been increasing since the first observational survey for NHTSA in 2002, especially on major arterials that are not access-limited (NHTSA, 2018). Higher speed means that passenger vehicle compartments have diminishing occupant protection given the forces released in high-speed crashes (IIHS, 2021a). The trend of increasing vehicle size (while providing greater occupant protection) means higher kinetic energies are also absorbed by others involved in crashes with larger vehicles (IIHS, 2021b). In addition, the power to weight ratio of vehicles has been increasing in auto models over time. Other important factors being equal, people driving more powerful vehicles were more likely to exceed the speed limit in an observational study by IIHS (McCartt & Hu, 2017). Drivers of more powerful vehicles also preferred going faster by self-report (Yannis et al., 2013). Furthermore, new driver support features such as adaptive cruise control may not always work as intended, potentially contributing to increases in speeding (Monfort et al., 2022). Socioeconomic and cultural norms also continue to glorify speeding. Motor vehicles are marketed using speed and have capabilities that far exceed any legal limits of even the highest speed highways.
Drivers also report high percentages of speeding (Schroeder et al., 2013). In NHTSA’s (2018) survey of driver attitudes and speeding behaviors, 30% were classified as “frequent” speeders, 40% as “sometime” speeders, and 30% as “non-speeders.” Driver beliefs sometimes seem at odds with their behavior. For example, two-thirds of drivers strongly agreed that “it is unacceptable to exceed the limits by more than 20 mph,” and 91% agreed that “everyone should obey the speed limit because it’s the law.” Yet 82% agreed that “people should keep up with the flow of traffic,” and 51% agreed that speeding tickets have more to do with raising money than they do with reducing speeding. Speeders also tended to be younger compared to non-speeders and sometime speeders, and to view the need to do something about speeding as less important. Across all drivers, however, 87% of surveyed drivers thought it was very important (48%) or somewhat important (39%) that something is done to reduce speeding.
Drivers engage in speeding for many reasons and may value speeding at a personal level, even though they value safety and want government to do something about speeding (Mouter et al., 2018). Most drivers believe speeding is a safety problem and that the government has a duty of care regarding the transportation network, whereas individual drivers are strongly motivated to reduce their personal travel times (Mouter et al., 2018). Drivers also do not individually pay other costs of speeding such as increased external noise or air pollution and so are unlikely to consider these costs in selecting speed, despite the fact that many of these additional costs increase as operating speed increases (Elvik, 2018).
Much of driving takes place at a pre-conscious level. Contrary to widespread belief, drivers often are not consciously trading-off perceived risk with perceived rewards (and continuously) selecting driving speed. Humans tend to learn from experience of road and traffic conditions and develop cognitive “scripts” or routines that simplify the driving task (Dumbaugh et al., 2020; Theeuwes et al., 2012). As a result of this type of pre-conscious response (or due to simply not being aware of the speed limit), many speeding episodes may occur unintentionally or incidentally as found in a naturalistic study of driver speeding behavior (Richard et al., 2012).
Roadways are designed for specific operational purposes, but changes in land development and roadside context can cause discrepancies between the intended purpose of a roadway and its actual function. For example, higher speeds tend to be anticipated in rural contexts, but as land adjacent to rural or suburban roadways are developed to provide more access to residential or commercial facilities (or to accommodate the needs of other kinds of road users), the originally intended operating speed on the roadway may no longer be appropriate (Stamatiadis et al., 2018). The posted speed limits and the actual operating characteristics of the road—as indicated to drivers by the road profile, the roadside development, and the presence of other road users—both influence perception of appropriate or safe driving speed (when traffic density isn’t controlling). These perceptions and preferred speed may differ from actual speed limits (Goldenbeld & van Schagen, 2007; Lee et al., 2017; Mannering, 2009; Yannis et al., 2013). Although speeding-related fatalities are highest on urban, non-interstate roads (NCSA, 2022), drivers also tend to wish for higher speed limits in such areas (Yannis et al., 2013), and frequently do not comply with lower urban speed limits.
Speed is an important element of roadway design because it directly links to other specific geometric features (e.g., curve radii) and because it can be influenced by roadside features (AASHTO, 2018). While roadways are designed and built to nominal standards and to accommodate human factors, in the United States and abroad, these designs historically did not always account for human injury tolerance (Peden et al., 2004) and some speeds that are safe and appropriate in some contexts (like straight segments on freeways) may result in unsafe operating conditions or crash energies that exceed human injury tolerance in other contexts (such as at horizontal curves or where pedestrians and bicyclists are present). The research literature verifies that systemwide crash risk is associated with specific environmental conditions—including the road’s cross-section, and roadside elements, like building setbacks—that may contribute to crash-producing mistakes (Dumbaugh & Li, 2011). Performance metrics, however, used to identify problems related to congestion may lead to designs that actually increase the perceived operating speed drivers interpret as safe but actually may not be suitable or appropriate (Lee et al., 2017; Kumfer et al., 2019). Some agencies are now moving away from the kinds of land use analyses (such as assessing vehicular delay) that trades congestion mitigation for safety impacts (Caltrans, 2020). This shift in priority aligns with the U.S. DOT’s (2022) goal of prioritizing safety and safe speeds over vehicular throughput.
Some drivers, of course, consciously choose to engage in speeding behaviors because of time pressures, perceived low risk of detection, or perceived high enforcement thresholds (Mannering, 2009; Schechtman et al., 2016; Yannis et al., 2013). Drivers also engage in speeding behavior to conform with the speeds of others (Åberg et al., 1997; Yannis et al., 2013). The amount of annual miles driven has also been correlated with more speeding (Yannis et al., 2013). Some speeding and aggressive driving may also arise among those who enjoy driving fast (Yannis et al., 2013), and from persistent driver attitudes and personality traits (Richard et al., 2016). Drivers who engage in frequent speeding behaviors also tend to take other risks such as driving at higher maximum speeds than other drivers and high-speed variability (compared to other drivers), tailgating, cutting off other drivers, and other risky driving behaviors when in a hurry (Richard et al., 2016).
Aggressive driving may involve driver anger, attempts to gain an advantage over other drivers, and deliberate violations and deviations from normal traffic speeds (Neuman, Pfefer, Slack, Hardy, Raub, et al., 2003). A predisposal to styles or habits of driving that frequently puts others at risk might qualify as aggressive driving and may be the norm for a small proportion of drivers, while others may be provoked to drive aggressively on occasion by exceptional congestion and when time pressures are greatest (such as during rush hours) (Shinar & Compton, 2004). Other drivers’ actions are also sources of irritation for “reactive” style drivers who may respond aggressively to being impeded by other drivers or to others’ reckless driving or actions perceived as hostile (Björklund, 2008). Other life stressors, such as combat deployments, may also contribute to aggressive driving (Mitra-Sarkar & Andreas, 2009). Others may simply lack impulse control, maturity, or a sense of social responsibility (Fuller et al., 2008). As discussed in the Increasing Penalties countermeasure section, these types of drivers also tend to be most difficult to deter through laws, enforcement, and punishment and may warrant other types of interventions.
Speeding is truly a complex safety problem involving the intersection of socioeconomic norms and trends (including vehicle preferences, consumer trends and others); balancing mobility and safety expectations; design of road networks, land uses, and changes among these; human behavior and fallibility; and legal frameworks, enforcement, and adjudication practices. There are many potentially effective policies, practices and tools that may be restricted, limited, and lacking in political support. For example, the expectation is that drivers should be able to control their own speed, but as research has shown, drivers often lack the ability or the will to always limit speeding on their own. Policies for widespread implementation of technologies such as Intelligent Speed Assistance that could offer the kind of control that drives do not want or are unable to exact for themselves have not garnered the necessary research or political backing in the United States, although they have been widely tested and are being implemented in Europe and other countries that have adopted and implemented the Safe Systems approach. Further, these measures are less likely to be adopted and consistently used on a voluntary basis by those who need them most.
Traffic enforcement in many jurisdictions has proven difficult to sustain (Thomas et al., 2013), and the situation may be getting worse. Surveys suggest that there is decreasing enforcement relative to vehicles miles traveled, and there may be less use of high-visibility traffic enforcement due to manpower issues in some jurisdictions (Byrne et al., 2021). High-visibility enforcement models also may face new challenges from cell phone apps available to alert drivers of radar and speed safety camera (SSC) locations allowing for route changes or speed reductions in the specific areas to avoid citations. In addition, the use of proven technologies such as SSCs that could be used to magnify enforcement and significantly increase general deterrence of speeding have been highly restricted in many States (NTSB, 2017). (See the Speed Safety Camera Enforcement countermeasure.)
Traditional traffic enforcement faces other challenges and feedback loops that limit durability and effectiveness. For example, speed enforcement may be increased due to a speeding and crash problem. When speeding declines, the enforcement may be reduced and the resources directed elsewhere, with the result that speeding soon increases again. Regular traffic enforcement and adjudication also allows significant discretion to law enforcement officers and the courts, which while intended to allow judgement and situational considerations, may lead to different outcomes for different violators. As more citations are issued, a backlog of court cases may result in ineffective and inconsistent prosecution if prosecutors and judges do not believe in the seriousness of speeding cases and sufficient resources are not provided (TRB, 1998; Alexander & Stradling, 2021b). These practices may the effectiveness and perceived legitimacy of speed enforcement as a safety issue (Neuman et al., 2009) and may even contribute to less enforcement (Byrne et al., 2021).
These inter-woven conditions explain why it is essential to have a comprehensive and coordinated, system-based approach to managing speed—one that incorporates local and State input from a range of stakeholders. Many engineering countermeasures are available to address speeding, and these measures may be more effective long-term and easier to implement than enforcement. Furthermore, speed lowering designs should be considered alongside changes to speed limits to help achieve speed limit credibility. Many of the countermeasures in this chapter also require involvement of State DOTs because of their role in setting speed limits and evaluating the need for road safety improvements (which should be done before determining need for enforcement strategies). The Key Resources section provides links to guidance for developing such plans and strategies.