Speed Warning System Tested in Minnesota Reduces Vehicle Speed by up to 10 Percent on Curved Road Segments.

Field Test Finds that Drivers Felt Safe Using Speed Warning System Along Curved Rural Roadways.

Date Posted

In-Vehicle Dynamic Curve-Speed Warnings at High-Risk Rural Curves

Summary Information

Lane-departure crashes at horizontal curves represent a significant portion of fatal crashes on rural roads. This project evaluated the feasibility of an in-vehicle dynamic curve-speed warning system in the form of a smartphone/mobile app that included both a visual and an auditory component. A total of 24 participants took part in a controlled pilot study of the warning system at the Minnesota Highway Safety and Research Center from June 23, 2017 to June 27, 2017. Vehicle speeds at key points through the curve were analyzed to quantify the system’s impact on driver behavior and performance. Following the testing, researchers conducted qualitative analysis on participant responses to subjective questionnaires about system usability and perceived trust.


The test system was designed so that drivers would receive alerts from the smartphone app as they pass through three checkpoints when approaching curves on the test track.

  • Checkpoint 1: drivers would receive a curve sign with no audio;
  • Checkpoint 2: drivers would additionally receive an audio warning if they exceeded the speed limit, and the sign would turn red;
  • Checkpoint 3:  “reduce speed" audio warning would continue if drivers kept travel above the speed limit.

Participants completed six drives on a closed test track, in which the first and last drives were used to establish participants’ driving behavior, while the middle four were experimental runs, with each drive varying in how far the second checkpoint was from the entry point of the curve. 

After the first and last drive, participants self-reported mental effort and workload on a Rating Scale Mental Effort (RMSE), a scale with a range from 0 to 150 (with a score of 0 meaning “no effort”). After completing a treatment run, participants completed subjective Likert-style questionnaires asking them to rate the mental effort of the driving task, perceived system usability, and perceived trust in the system.

The System Usability Scale (SUS) was used to assess user satisfaction and willingness on a scale of 0 to 100 (with 0 meaning that the system was complex and difficult to use and 100 meaning it was easy to use and well-integrated).


  • Drivers approached horizontal curves 8 to 10 percent slower than when not using the system; this shows that drivers following the warning prompts successfully lowered their speeds as compared to when no warnings were present.
  • Drivers’ mental effort did not differ significantly between the baseline drives and treatment with adequate braking distance, meaning that the curve warning system does not negatively impact driver workload.
  • The treatment where curves were about 43 ft. apart was significantly more navigable than all other treatments. It received the lowest RSME score (37.4) and highest SUS score (88.85). The warning system deployment (relative to the curve and advisory curve speeds) on this treatment was also the most preferred.
  • The Likert-scale questionnaire results found that participants rated the system as a 4.68 out of 5 for making them feel safer if other drivers used the system and a 4.5 / 5 for willingness to recommend the system to others.
Results Type
Deployment Locations