In Salt Lake City, Utah the ADVISE fog warning system tested on a two-mile section of I-215 promoted more uniform traffic flow, reducing vehicle speed variability by 22 percent while speeds increased 11 percent.
Made Public Date


Salt Lake City
United States

Adverse Visibility Information System Evaluation (ADVISE): Interstate 215 Fog Warning System, Final Report.

Summary Information

During the 1995 to 2000 winter seasons, a technology known as the Adverse Visibility Information System Evaluation (ADVISE) was tested on a two-mile section of I-215 subject to recurring fog in Salt Lake City, Utah. The purpose of the system was to reduce the variation in road speeds and provide a more uniform traffic flow during fog. The system used four roadway visibility sensors, a central computer system, wireless communication devices, and two roadside dynamic message signs to communicate speed recommendations to freeway travelers. The visibility sensors were installed on low lying sections of roadway to measure sight distance every 60 seconds. This information was communicated to a central computer that used a weighted average algorithm to evaluate visibility conditions and post ADVISE messages on roadside dynamic message signs.

The following ADVISE messages were posted as fog conditions changed. The speed limit was 65 mi/h.

Highway Visibility Range ADVISE messages on DMS
>820 feet No Message
650-820 feet "Fog Ahead"
490-650 feet "Dense Fog” alternating with "Advise 50 mi/h"
330-490 feet "Dense Fog” alternating with "Advise 40 mi/h"
200-330 feet "Dense Fog” alternating with "Advise 30 mi/h"
<200 feet "Dense Fog” alternating with "Advise 25 mi/h"


To evaluate the impacts of the system, in-pavement loop detectors were used to monitor vehicle speed and classification by lane, direction, and time of day during foggy and clear conditions, before and after the system was deployed. Evaluation data were collected in three phases. Phase I data were collected prior to deployment to evaluate traffic conditions when drivers made their own decisions about travel speeds (baseline condition). Phase II data were collected to calibrate the system, and Phase III data were collected to estimate system impacts.

Between the Phase I and Phase III data collection periods, I–215 was re-striped and changed from a three-lane to a four-lane highway to service traffic detouring onto I-215 as a result of I-15 ramp closure projects. The presence of peak flow traffic conditions was assumed to alter vehicle speeds as much as the presence of fog; therefore, to help isolate the influences of the advisory speed messages, data collected during peak periods were not included in the analysis.

The Phase I data collected on February 3, 4, and 6, 1996 represented 18 fog events, 594 minutes of adverse weather, and 38,522 individual vehicles traveling in off-peak periods. The Phase III data collected on December 30, 31, 1999, and January 1, 2000 represented three fog events, 152 minutes of adverse weather, and 6,803 individual vehicles traveling in off-peak periods. Although the number of vehicles observed was considerably lower in the "after" condition, the study findings were shown to be statistically reliable.


The results indicated the deployment was successful at promoting more uniform traffic flow during fog events. As a surrogate measure of safety, the improved uniform traffic flow indicated there was less risk for drivers traveling in recurring fog zones. The data showed that when recommended travel speeds were provided, the number of excessively slow drivers decreased. The ADVISE technology effectively reduced the average standard deviation of speed between vehicles by 22 percent. Prior to the deployment, the standard deviation was 9.5 mi/h. After the system was deployed and ADVISE messages were provided, the standard deviation decreased to 7.4 mi/h.

The report also noted that the average vehicle speed measured during fog events increased from 54 miles per hour (mi/h) to 62 mi/h after the system was deployed. The report concluded that the increase in average speed was indicative of the overall 6 mi/h increase in average speed observed during non-fog days since 1996, and was partly attributable to the reduction in excessively slow drivers during fog events.

The author noted that the post-deployment evaluation period was relatively limited, and it was highly probable that this type of system was new to many drivers.


See also:

Perrin, Joseph, et al. Effects of Variable Speed Limit Signs on Driver Behavior During Inclement Weather. Paper presented at ITE 2000 Annual Meeting. Nashville, Tennessee. 6-10 August 2000.