Speed Management Rationale 

The primary reason for regulating individual speed choices is the significant risks drivers can impose on others.  For example, a driver may decide to drive faster, accepting a higher probability of a crash, injury, or even death in exchange for a shorter trip time.  This driver’s decision may not adequately take into consideration the risk this choice imposes on other road users.  This imposition of risk on others, and the desire to protect public safety, are the primary reasons for the government’s role in setting speed limits.

Another reason for regulating speed derives from the inability of some drivers to correctly judge the capabilities of their vehicles (e.g., braking, steering) and to anticipate roadway geometry and roadside conditions sufficiently to determine appropriate driving speeds.  This reason may not be as relevant for experienced motorists driving under familiar circumstances.  However, inexperienced drivers or experienced drivers operating in unfamiliar surroundings may underestimate risk and make inappropriate speed choices.  Even drivers familiar with a particular road can make inappropriate decisions because of fatigue or other factors.

A final reason for regulating speed is the tendency of some drivers to underestimate or misjudge the effects of speed on crash probability and severity.  This problem is often manifested by young and inexperienced drivers and may be a problem for other drivers.  The risks imposed on others and the adequacy of information about appropriate driving speeds vary by road class.  For example, the risks imposed on others by individual driver speed choices are likely to be relatively small on rural Interstate highways where free-flowing traffic creates fewer opportunities for conflict with other road users or roadside obstacles.  Moreover, under normal conditions, drivers typically have adequate information to determine appropriate driving speeds because these highways are usually built to the highest design standards, access is limited, and roadside activity is minimal.  In contrast, the risks imposed on others by individual driver speed choices may be large on urban arterials where roadside activities are numerous and traffic volumes are high for extended periods of the day, increasing the probability of conflict with other road users.  These differences are important factors for consideration in setting appropriate speed limits on different types of roads.