Acknowledgements

The research presented in this paper was supported by funding from the Mountain Plains Consortium (U.S. Department of Transportation) and the Utah Department of Transportation through the Utah Transportation Center.

Disclaimer

The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the information presented.  This document is disseminated under the sponsorship of the Department of Transportation, University Transportation Centers Program, in the interest of information exchange.  The U.S. Government assumes no liability for the contents or us thereof.

Executive Summary

This study examines signal timing in inclement weather conditions.  With the completion of the Utah Department of Transportation's (UDOT) Advanced Traffic Management System (ATMS), UDOT will have the capability to change signal timing plans from a central location by communicating with each controller through fiber optic technology.  With this ability, it has become feasible to have a library of special signal timing plans.  One of these plans could be for inclement weather conditions.  This research reviews relevant literature for similar studies, interviewing other state departments of transportation, and collecting local traffic flow data to determine traffic flow characteristics during inclement weather.

Traffic flow data is collected over a range of seven inclement weather severity conditions at two intersections:  700 East / 900 South and 1300 East / 500 South. The results of the data collection indicate that the largest decrease in vehicle performance occurs when slush begins to accumulate on the road surface.  After this critical point, saturation flows decrease by 20 percent and speeds decrease by 30 percent.  Start-up lost times also increase by 23 percent.  These results are compatible with those found in similar research.  Few other state departments of transportation have considered modifying their signal timings in inclement weather.  While Minnesota and Alaska both have done research in the area, Alaska is the only known state that currently has a signal timing plan for inclement weather conditions.

Based on the collected data and the relevant research, it is recommended that UDOT develop and implement modified coordinated signal timing plans for the major signalized corridors in the Salt Lake Valley.  The plans can easily be developed by modifying the data used to create standard "dry" signal timing plans and by changing a small number of other parameters.

It is further recommended that signal timing plans be manually "switched on" by a trained operator or engineer once an alarm identifies the potential need.  The trigger for the alarm may include observed reductions in travel speeds by mid-block detectors or reduced saturation flows from stop bar detectors.  Four general criteria must be considered to ensure that such a plan gives the maximum benefit.  First, the storm must be sufficiently severe to cause "inclement" road surface conditions.  Second, the storm duration must be predicted to continue to cause inclement surface conditions for at least 20 minutes, to allow for the negative effects of transitioning from one signal timing plan to another to be minimized.  Third, the storm must affect a sufficient length of corridor to benefit the majority.  Thus, many microclimate events that only affect a small geographical area should not initiate the new timing plans.  Finally, the traffic volumes must be substantial enough to warrant the time to switch a plan.  A.m. and p.m. peak hours will be the most likely candidates, while off-peak plans on specific corridors should be individually considered.  Special attention should be given to individual intersections that have high speed or steeply graded approaches, as they will cause additional problems in low traction conditions.