Chapter 6. Conclusion and Recommendations

Our research investigates the effect of inclement weather on signalized traffic in Salt Lake City.  Saturation flow, speed, and start-up lost times were measured during various weather severity levels at 700 East at 900 South and 1300 East at 500 South in Salt Lake City.

Based on current literature, contact with other state DOTs, and our own data collection, we conclude following:

• Saturation flows and speeds are reduced and start-up lost times are increased with severity of the road conditions.  The greatest decrease in these values is found when slush begins to accumulate on the road surface.
• Several other studies examine the behavior of traffic signal timing systems during inclement weather or related topics.  Most cold-weather states, however, do not modify their signal timings during inclement weather.
• A special signal timing plan should be implemented on major corridors throughout the Salt Lake Valley.  Although this plan could be justified on safety considerations alone, such a plan should also provide some operational improvement (Maki 1999).

It is recommended that UDOT develop and use a signal timing plan for use in inclement weather.  This plan easily can be developed, based on the collected data used to develop existing timings.  The new signal timing plan should have the following characteristics:

• The plan should have new splits and offsets.  Cycle lengths should remain the same unless inclement weather specific traffic counts are provided and indicate a different cycle is more appropriate.
• Increase amber time by 10 to 15 percent (one-half to one second) depending on intersection size.  Half-second increase for intersections under 50 feet wide increasing to a one-second increase for intersections 100-feet wide.
• Increase all red time by one second to account for the slower clearing of the intersection by sneakers at permitted/protected intersections (taking 0.75 seconds longer than during dry conditions).
• Decrease the measured or calculated dry saturation flows by 20 percent
• Decrease the average dry speeds by 30 percent
• Start-up lost time should increase by 23 percent from 2.0 to 2.5 seconds.

It also has been determined:

• Gap acceptance time appears to increase by 25 to 30 percent, but this is an opinion as gap acceptance was outside the scope of this study.
• No change in pedestrian crossing timing needs is necessary based on prior research.
• Clearance intervals may be increased even further at intersections that have high speed or steep grade approaches.

The inclement weather signal timing plan should be activated based on engineering judgment.  Four general areas should be considered when making this judgment to ensure that the plan provides sufficient benefit to the network.  These areas are: severity, duration, area of influence, and traffic flows.

1. Severity.  There should at least be slush on the road.  Another measure of the severity might be to check on the current speeds.  If the speeds fall below about 70 percent of normal (due to weather conditions), this could be used as a consideration for implementing a plan.  Such information may be able to be derived from the TOC speed map.
2. Duration.  The predicted duration of the storm must be sufficient to warrant the plan.  Translating from one plan to another causes the network to be unable to recover quickly enough to benefit from a plan (Gartner, Stamatiadis, and Tarnoff 1995).  It is therefore recommended that the storm be projected to continue to cause poor road conditions for at least 20 minutes to allow the network to recover from the transition.  This will justify the plan's disruption to traffic flow in the form of transition from one plan to another.
3. Area of Influence.  The storm must influence a sufficient area.  If the plan can be implemented by corridor, then the storm must be affecting a sufficient length of the corridor to be effective.  What this length is will depend on engineering judgment at the time.  This is an important part of the warrant because of the lake effect snowstorms that frequent the area.
4. Traffic Flows.  Traffic conditions should be sufficient to warrant a new timing plan.  These signal timing plans probably are most useful on a corridor basis, rather than a network-wide application.  A.m. and p.m. peak plans are the most evident for which plans should be most effective.  Mid-day plans also could be developed, depending on the demand and the perceived benefit the network or corridor might receive.  Night or weekend plans may not receive as much benefit because of the decreased demand.

Attempt to determine the potential operational benefit from modeling and simulation are underway on sample downtown Salt Lake City corridors.  This information will be provided in the form of a supplemental report upon completion.

References

Bernardin Lochmueller and Associates, Inc.1995. Anchorage Signal System Upgrade Final report Botha, J.L. and T.R. Kruse. 1992. Flow Rates at Signalized intersections Under Cold Winter Conditions.  Journal of Transportation Engineering pp.439-450.

FHWA Report. 1977. Economic Impact of Highway Snow and Ice Control, Final Report. FHWA-RD-77-95.

Garber, N. J., and L. A. Hoel. 1988. Traffic and Highway Engineering. St. Paul: West Publishing Company.

Gartner, N. H., C. Stamatiadis, and P. J. Tarnoff.  1995. Development of adaptive signal control strategies for intelligent transportation systems:  a multilevel design.  Presented at the 74th annual meeting of the Transportation Research Board.

Gillam, W.J and R.A. Withill. 1992. UTC and Inclement Weather Conditions. IEE Conference pp.85-88

Jones, E.R., M.E. Goolsby, and K.A. Brewer. 1970. The environmental influence of rain on freeway capacity. Highway Research Record 321, HRB, National Research Council, Washington, D.C., pp.74-82.

Knoblauch, Pietrucha, and Nitzburg .1996. Field Studies of Pedestrian Walking Speed and Start-Up Time Transportation Research Board no.1538 pp. 27-38.

Maki, Pamela J. 1999.  Adverse Weather Traffic Signal Timing, P.E.  69^th Annual Meeting of the ITE, Las Vegas.

Parsonson, Peter S. 1992. Flashing Operation and Signal Timing for Adverse Weather.  NCHRP Synthesis of Highway Practice pp. 46-47.