Skip to main content
You can also sort pages by filters.
Table of Contents
Download the Full Book

Effectiveness: 3 Star Cost: Varies
Use: Unknown
Time: Varies

A key premise of speed management is that drivers must at all times and locations know what the speed limit is in order to be able to obey that limit. With the complex task of driving and navigating in traffic as well as policies that may encourage minimal posting of speed limit signs (or no posting in areas under default, statutory limits), drivers may need more help knowing the speed limit.

Intelligent speed assistance or intelligent speed adaptation (ISA) involves in-vehicle technologies that use GPS data interacting with accurate, digitally mapped speed limit data for the entire network or vehicle-based speed limit sign recognition. ISA systems can vary from minimal systems that provide information to active speed limit control that could be mandatory or voluntary (i.e., with on/off activation switches). Systems may:

  • Provide information only (display the speed limit and changes);
  • Provide visual or audible alerts when the speed limit is exceeded, but the driver can decide how to react (termed open system);
  • Provide accelerator resistance to make speeding more difficult, but still possible (termed half-open). This system is like cruise control, except the speed limit (not the driver) determines when to engage speed resistance. Drivers may be able to turn off the system with a switch; and
  • Automatically prevent speeding above the speed limit (mandatory speed compliance).

Compared to speed governors, which can only limit the maximum speed of vehicles, ISA has the potential to help control speed of all motor vehicle types according to the prevailing limit at a location.


While ISA has received limited trials in the United States, several vehicles with optional ISA advisory capabilities (open ISA level) are currently available (NTSB, 2017), and ISA systems that limit gas flow to engines are also available on a limited number of newer models (IIHS, 2021a). Internationally, the European Union is further along, having passed legislation in 2019 that will require all new vehicles sold starting 2024 to be fitted with ISA (European Commission, 2021). According to a summary on the SAE International (Visnic, 2019) website, European ISA will work much like cruise control to prevent the vehicle from traveling in excess of the posted speed limit by limiting engine power (and not through automatic braking). The systems will, however, be equipped with on/off switches to encourage public acceptance, but the default status will be “on” each time the vehicle is started. The systems can also be overridden by the driver, who remains in control of the vehicle speed. The EU also updated other vehicle safety measures, including a requirement for event data recorders to show whether the system was being over-ridden at the time of the crash.


ISA has been found to lower speeding among drivers using the systems. Varied types of systems have been widely studied in European countries for acceptability and effects on driver behavior. In Europe, the effects on speeding have been dramatic for both warning and control type ISA systems, decreasing the amount of speeding and narrowing the speed distributions while the systems are being used (Biding & Lind, 2002; Carsten, 2012; Lai & Carsten, 2012; van der Pas et al., 2014; Várhelyi et al., 2004). Like other speed control measures, there seems to be little potential for a lasting educational benefit or “training” of drivers to control their speed once the systems are deactivated (Chorton & Conner, 2012; van der Pas et al., 2014; Várhelyi et al., 2004). In a long-term study of 284 drivers using an active accelerator pedal, Várhelyi et al. (2004) found that while driver speeds increased somewhat from initial drops, speeds remained lower over the long-term (5 to 11 months), remained lower when the systems were active, and were significantly lower than that of other drivers. Furthermore, emissions were reduced, and travel times were not significantly increased.

Based on relationships between speed and safety, substantial crash savings are expected (van der Pas et al., 2014). Assuming that the risk of speeding, and speeding-related fatalities and injuries could be virtually eliminated among those using the systems, Vaa et al. (2014) estimated that ISA was the most effective driver support system for saving lives and reducing injuries compared to speed governors, adaptive cruise control, electronic stability control, and other non-speed-related driver systems such as alcohol interlock technologies. The European Transport Safety Council estimated that the upcoming fitting of ISA on all cars, vans, buses, and heavy goods vehicles will eventually cut road deaths by 20% across the European Union countries.

In U.S. trials involving young drivers and repeat speeding violators, results have also found reductions in speeding and compliance with speed limits. Researchers developed and pilot-tested a half-open system (increased accelerator counter pressure) when the limit was exceeded by young drivers 18 to 24 years old22 in the experimental group (with system on for half the drives) and 22 in the control group with no ISA (Blomberg et al., 2015). The experimental group showed less speeding on drives when the ISA was activated compared to not activated and compared to the control group. The researchers also assessed user perceptions of the performance and “likability” of the system. In general, performance was rated more highly than likeability-related factors. De Leonardis et al. (2014) also found significant reductions (22%) in the average amount of trip distance that 78 Maryland drivers with prior speeding citations sped during a 4-week treatment period compared to baseline driving. Only alerts were given in this system. However, as in other trials, speeding began rebounding within 2 weeks of alerts being discontinued. In another U.S. study of 85 teen drivers randomly assigned to different groups, the only condition that achieved reduced speeding behavior was when an in-vehicle system alerted the teens to the speeding, parents received reports on the unsafe behaviors, and teens believed the parents would not be notified if they corrected the speeding (Farmer et al., 2010). Again, there were no lasting effects on speeding behavior once the systems were inactivated.

There is a need to provide current and accurate maps of speed limits (Carsten, 2012), and to have reliable speed limit sign-reading camera and related technologies (NTSB, 2017), as well as to have the technologies installed in new vehicles or retrofitted to older vehicles. Speed limit maps currently must be purchased by vehicle owners in the United States. According to information provided to the NTSB by vehicle manufacturers providing ISA systems, updated maps may not be regularly purchased by U.S. owners (and maps may also be updated only once per year) (NTSB, 2017). Newer technologies could also make use of speed limit sign-reading technologies.

Other uncertainties still exist about the rate of driver adoption of the technologies, driver behaviors or potential adaptations over time, and even external forces that may potentially affect the costs and benefits of ISA (van der Pas et al., 2012). For example, only about half of Swedish drivers in one study indicated they would accept a non-driver controlled ISA system in their cars, whereas a majority would accept it as a driver-controlled system (Várhelyi & Mäkinen, 2001). Other evidence suggests that prospects for wide-spread voluntary adoption of ISA, even with financial incentives such as reduced insurance premiums or vehicle price discounts, may be limited (Vaa et al., 2014).

Estimates of speed and safety relationships suggest widespread or universal deployment of ISA could lead to significant crash, fatality, and injury reductions. The largest safety benefits for various types of potential ISA roll-outs, would likely be for systems requiring mandatory speed limit compliance that cannot be over-ridden by the driver or turned off (Carsten & Tate, 2001). In addition, dynamic systems that could react to variable conditions such as surface friction and traffic, as well as fixed and variable speed limits, could yield maximum crash savings – up to 15 times the cost to implement and run the system per some estimates (Carsten & Tate, 2001). Requiring ISA for higher risk groups (younger drivers, professional drivers, drivers convicted of serious speeding offenses) was deemed most cost-effective among partial ISA roll-outs (Vaa et al., 2014).

Other considerations:

  • Voluntary (driver-controlled) systems: Relying on voluntary adoption may not be sufficient to achieve consistent use by serious offenders or younger driver groups who may benefit most from ISA. Serious offenders were more likely to disable or over-ride ISA than other drivers (van der Pas et al., 2014). Younger drivers may also be less likely to adopt ISA, even with incentives (Chorlton et al., 2012; Lahrmann et al., 2012), although it is possible that the right level or type of incentives have not been identified. Once the systems are in use, feedback on rewards/incentives should be in real time. Also, while most previous violator participants in the Maryland study seemed to accept the ISA systems, they also expressed concerns about providing driving speed data to insurance or licensing agencies. They anticipated negative consequences, including the potential for revocation of their driver licenses and increased insurance premiums. Such concerns would need to be addressed to encourage drivers to voluntarily use such a system to help control their speed (De Leonardis et al., 2014).
  • Rewards: Although several field tests from Europe found that drivers exceeded limits less when offered economic incentives such as reduced insurance premiums or discounts (for lease vehicles), the interaction of incentives with ISA or use of incentives alone to encourage safer drivers has met with mixed results. Incentives combined with ISA or ISA alone contributed to less speeding among a group of Danish volunteers, but incentives alone had a small effect (Lahrmann et al., 2012). Furthermore, the researchers were unable to recruit sufficient volunteers based on the promise of these incentives. Conversely, Reagan et al. (2013) found that a modest monetary incentive alone, when displayed in real time in the vehicle to an already recruited sample of younger U.S. drivers, achieved significantly more compliance with speed limits. A system of increasing intensity of alerts alone also decreased speeding. The study excluded drivers with less than 5 years driving experience or with prior convictions or suspensions for impaired or reckless driving (arguably the groups most in need of the intervention).
  • In other studies of incentives, lease cars in the Netherlands were equipped with technology that continuously monitored and displayed whether drivers were allowing a safe following distance and complying with the speed limit but did not otherwise intervene. Rewards were given by the lease company for good driving behavior over a 16-week period. Drivers were about 20% more likely to drive within posted speed limits and 25% more likely to maintain adequate following distances when receiving feedback and rewards (Mazureck & van Hattem, 2006). A pay-as-you drive plan to save young drivers’ insurance costs also reduced the percentage of miles that young drivers exceeded the limit by 14% (Bolderdijk et al., 2011).
  • Related technologies: Adaptive cruise control (ACC) works similarly to standard cruise control, except that, in addition to maintaining a speed set by the driver, a radar system in the front of the vehicle detects and responds to other vehicles in the lane ahead to maintain a safe following distance. A study involving 40 Boston area drivers found that drivers using ACC were more likely to speed on interstates and freeways, and by a higher degree than when manually controlling the vehicle’s speed (Monfort et al., 2022). The vehicles in the study allowed the drivers to easily modify their speed up or down by 5 mph using a toggle control, which may have contributed to the results observed. These findings highlight the importance of details, and human adaptation to the way systems are designed as well as voluntary efforts to control speeding behavior. ITS may not suffer from these effects since the vehicle detects the speed limit from signs or from speed limit geo-databases. However, designers, evaluators, and policymakers should be attuned to the potential for other unintended adaptations or effects.
  • Need for political action: The main roadblock to implementation may be political, rather than safety or technological reasons (Carsten, 2012). Policymakers are ever attuned to public perception and pushback. More State and national-level discussion and communication about the dangers of speed, and the most effective strategies for controlling speeding may be needed.