In response to a citizen complaint, a local traffic authority requested that the State DOT conduct an engineering study and consider lowering the existing 45-mile-per-hour (mph) speed limit to 40 mph on a U.S. route within a town. The State DOT completed a field review of the section of the route in question to gather information on roadside development, bicycle and pedestrian usage, curve measurements, and other geometric factors. It also reviewed nearby historical speeds, along with the latest available 3-year crash experience to determine the recommended speed limit. The results of the field study did not support changing the speed limit. To obtain an additional data point, the State DOT entered the relevant data into the FHWA web-based USLIMITS2 speed-limit-setting tool. The results of USLIMITS2 supported the field study findings that there was no need to change the speed limit.

Traffic engineers must often communicate with community leaders, government officials, and the public to explain the methodology behind setting, maintaining, or changing speed limits. USLIMITS2 is FHWA‘s free, web-based expert system tool that provides an objective second opinion and helps inform these speed limit decisions. Determining an appropriate speed limit is a complicated process that needs engineering judgement, as it takes into account combined conditions of roadway geometrics, traffic safety and operations, roadway users, and the environment. USLIMITS2 is a decision rule-based expert system, providing more consideration to and weighting of roadway factors by mimicking an expert’s thought and decision-making process.

USLIMITS2 was developed in 2008 through the National Cooperative Highway Research Program (NCHRP) Project 3–67, Expert System for Recommending Speed Limits in Speed Zones. Since then, it has been used by State and local agencies to augment the credibility of engineering speed studies, helping to address concerns from local government and private citizens when speed limits are unchanged or adjusted, depending on the situation.

USLIMITS2 was selected in 2018 as one of FHWA‘s 20 Proven Safety Countermeasures, which are infrastructure-oriented safety treatments and strategies chosen based on proven effectiveness and benefits. As a result, USLIMITS2 has been promoted by FHWA to encourage widespread implementation by State, Tribal, and local transportation agencies to reduce serious injuries and fatalities on America’s highways.

To date, agencies from 29 States and Washington, DC, have used USLIMITS2. State DOTs in Alabama, Florida, Texas, and North Carolina have recommended use of USLIMITS2 in their speed setting manuals, SHSPs, pedestrian/bicyclist safety action plans, and other safety documents.

USLIMITS2 User Feedback:

  • “It’s nice to have another data point to either confirm what we are already deciding to do, or to give us pause to look at things more closely if the program recommends something different than what we were thinking.”
  • “USLIMITS2 is a good tool to use to determine speed limits and not solely rely on engineering judgement.”
  • “USLIMITS2 is useful especially for local agencies that have limited tools and resources.”
  • “It is helpful to have all information entered into USLIMITS2 accessible in a report that can be saved in the project file.”

Since taking ownership of USLIMITS2 in 2012, FHWA has focused on ensuring that USLIMITS2 remains credible and useful. This includes making updates based on user feedback from workshops, webinars, and technical support emails. Enhancements include:

  • A more informative and intuitive home page.
  • More descriptive field names.
  • Enhanced ability to review and edit data before submitting.
  • Inclusion of the crash rate calculations in the final recommendation report.

An update to the tool is currently in progress to include a detailed description of how the speed limit is determined in the final recommendation report, which is produced after inputting information into USLIMITS2. This update is to address a particular user-raised issue: a so-called “black hole” on how recommended speed limits are generated by USLIMITS2, and helps users better understand and present the recommended speed limits from USLIMITS2.

In addition, the recently awarded NCHRP 03–139, Next Generation of the USLIMITS2 Speed Limit Setting Expert System, will focus on updating the speed limit recommendation decision rules to take into account the latest speed limit research and practices.

In March 2020, FHWA implemented a USLIMITS2 user group with the purpose of giving current and potential users a forum to discuss how they have used USLIMITS2, their issues and concerns, and their suggestions for updates. Future plans for the user group include quarterly webinars to discuss USLIMITS2 case studies and best practices, and creation of a steering committee to help identify questions for the group and respond to questions from members, as well as encourage participation in the user group.

To join the user group, send a blank email to USLIMITS2-subscribe-request@listserv.gsa.gov. If you are a regular user of USLIMITS2 and would like to serve on the steering committee, please email your interest to help@uslimits.org. FHWA offers free technical assistance to State and local agencies interested in learning more about using USLIMITS2. This includes answering questions, providing in-person and virtual workshops, and giving presentations about USLIMITS2. To request technical assistance or help with USLIMITS2 please email help@uslimits.org.