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Accident analysis of ice control operations.

SNOW and ice covered roads can paralyze communities and threaten public safety. They produce hazardous driving conditions, which increase traffic deaths, injuries, and property damage. Without close attention to the effective removal of snow and ice from roads, the economy of a region will suffer and traffic accidents will escalate. Most activities of individuals, industries, utilities, schools, and government activities are handicapped in social and economic ways during the duration of snow and ice conditions on roads and streets.

During the late 1980s, the Technical University of Darmstadt in Germany completed an extensive research project documenting the accident experience before and after winter road maintenance operations, and following a bare road pavement policy. The analysis of Horst Hanke and Christoph Levin covered over 4,700 accidents, about 1,900 casualties, on about 400 miles of roads outside built-up areas in four representative highway maintenance centers.

The cost of property damage was about $35 million. Salt was the only deicing material used on these roads. The time was determined at which salt spreading took place, then an hourly accident analysis was conducted for 12 hours before and 12 hours after. Results of this project are shown in Figure 1.

In Fall 1990 Marquette University was approached by the Salt Institute to undertake a similar study, utilizing the same methodology of the German study, but for one winter season, the winter of 1990 to 1991 instead of for four years as was the German study. The results of that research are reported here.

Methodology

This research was conducted for a doctoral dissertation submitted to Marquette University Graduate School by co-author Hanbali. Complete descriptions of the research methodology are contained in the original dissertation; here, only the briefest information of interest is provided.

Highway Testing Sections. A network of two-lane undivided and divided highways of about 520 miles and 50 miles, respectively, were selected randomly with the cooperation of authorities in New York (Wayne, Tompkins, Cortland, and Monroe counties), Illinois (Ogle and Lee counties), Minnesota (Rochester and Stewartville sub-areas), and Wisconsin (Walworth County).

The 570 miles of highway networks consist of a total of 24 divided multi-lane and 102 undivided two-lane highway sections. The highway testing sections selected for analysis were primarily rural or suburban and had traffic volumes from 1,000 annual average daily traffic (AADT) to approximately 30,000 AADT. Wherever a test section contained a variation in volumes that was considered multiple sections.

Traffic Volumes. Normal traffic volumes were calculated using accepted procedures. All 24-hour traffic volume counts provided by highway authorities in the four states were based on Monday through Thursday counts. These counts were used as the average day of the week (Monday to Thursday) traffic counts. A simple approach was used to decide if the day of the week conversion factors were required for weekend traffic volumes.

Hourly factors were generated from the hourly volumes provided or from an automatic traffic recorder (ATR) nearby. For every snowstorm, the hourly traffic volume was measured at an ATR station and compared to the normal hourly traffic volume for the same location during a similar day, and at the same hour, month, and year. From the comparison, hourly reduction factors were derived for the testing sections in that area during the snow storm in order to estimate vehicle kilometers of travel (VKT).

Time of Salt Application. Participating areas in New York, Illinois, and Wisconsin have been using salt as the common deicing material. At temperatures below 25 |degrees~ F participating areas in New York have been using mainly a mixture of salt, Ca|Cl.sub.2~, or in some cases abrasives-salt mixture when necessary. Wisconsin and Illinois have been using Ca|Cl.sub.2~ to prewet salt. The participating highway agencies in Minnesota have been using abrasive-salt mixtures independent of temperature range. In New York, a few events were selected in which salt and abrasives were mixed (a ratio of from 10 to 30 percent salt to total mix). However, no events using salt/abrasive mixtures were included in the following results.

For almost all two-lane highways, salt is usually applied to the middle third of the two lane pavement and the high side of super-elevated curves in all participating areas. In most cases the trucks spread salt one way and plowed on return or the reverse of this procedure depending on snow fall. The number of applications varies by condition. In all participating test areas, the average number of applications per average storm was 2.6.

In New York State, keeping a record of each snow and ice control field operation is part of the participating highway maintenance agency work activities. A copy of the snow and ice control records, which contain information such as route number, section code, start and end time of the snow and ice field operation, section length, type of action, and quantity of deicing material used, was provided for each snowstorm. Records containing date of the snowstorm, start and end of the snowstorm, start and end time of work, temperature range, type of snow, and depth of snow were also provided for each snowstorm by the authorities in each participating area in New York State.

In other participating areas a simple form was designed to serve the purpose of this research project. Route number, section code, start and end time of the snow and ice field operation, section length, and type of action were reported on the form. Records containing date of the snow storm, start and end of the snow storm and end time of work, temperature range, type of snow, and depth of snow were provided on a separate form for each month of the researched period.

The resultant combination of test sections (each with a uniform volume) and events (winter maintenance activity periods) resulted in sub-events that totaled 4,600.

Compilation of Accident Rates. The methodology for accident rate calculations was as follows:

1. For each testing section, the last salt spreading time was determined during each snowstorm (zero hour).

2. For each testing section, hourly intervals were taken backwards and forwards (up to 12 hours) from the spreading time (zero hour).

3. For each testing section, traffic volume and accidents were compiled separately for each hourly interval.

4. For each testing section, vehicle kilometers of travel (VKT) were calculated for each hourly interval.

5. For all testing sections VKT and accidents of the same relative hourly interval were totalled and traffic accident rates calculated in accidents per million vehicle kilometers of travel (MVKT).

Before and After Accident Analysis. Three analyses of before and after accident rates were conducted to determine the significance of difference using sound statistical methods. All three tests showed significant difference for the before and after accident rates for each hourly period "after" selected (from 1 to 12 hours) when compared to the equivalent hourly period (1 to 12 hours) "before" on two lane highways at the 99 percent confidence level. Because the sample size and hence accident frequency was smaller on freeways, the same hourly comparison showed a significant difference only for the first four hours, and only at the 95 percent confidence level, still significant however.

Benefit/Cost Analysis. A benefit/cost analysis of snow and ice control operations using salt only as a deicer was performed. The analysis used only the direct benefits of accident reduction, savings in vehicle operating costs and travel time, and direct costs.

Conclusions

Results very similar to those in Germany occurred in this study. Differences between the highway network utilized in Germany and in the U.S. resulted in differences in the shape of the hourly accident rate curve. The reason for the difference is that in Germany the maintenance districts analyzed had responsibility for all roads including local roads. Hence there was a longer time period involved from the beginning of snowfall to completion of the entire operation. Local road priority followed arterials. In this research study most two lane highways (except approximately one-third of the test sections in Walworth County, Wisconsin which were county trunk highways) were state highways. Hence they had a higher priority. From observations made by the researchers, the time of applications closely followed the time of the ice conditions on the highway, since all the test sections were major or minor arterials (or in the case of Walworth County, Wisconsin, some collectors) and none were local roads.

Conclusions are summarized under the appropriate highway type. All rates are in accidents or injuries per million kilometer miles of travel (MVKT).

Two Lane Highways. Winter maintenance, using salt or salt and Ca|Cl.sub.2~ only as deicers, reduced accidents as follows (comparing only 4 hours before and after):

* The rate for all traffic accidents before salt spreading is about eight times higher than that after (7.90 accidents/MVKT before and 1.02 accidents/MVKT after).

* The rate for injury traffic accidents is about nine times higher than that after (4.96 injury accidents/MVKT before and 0.58 injury accidents/MVKT after).

* The rate for property damage (PD) traffic accidents is about two times higher than that after. (2.94 PD accidents/MVKT before to 0.45 PD accidents/MVKT after).

* The severity of traffic accidents is reduced. The ratio of injury to property damage accident rates before (4.96/2.94 or 1.68) is about 30 percent higher than the same ratio after (0.58/0.45 or 1.29).

Winter maintenance reduced traffic accident costs from "before" to "after" by 88 percent and reduced the average cost of an accident by 10 percent.

* The traffic accident costs rate before was $388,200/MVKT ($625,000/MVMT) or 38.8 cents/VKT (62.5 cents/VMT) and the cost rate after was $45,963/MVKT ($74,000/MVMT) or 4.6 cents/VKT (7.4 cents/VMT).

* The average cost of an accident before was $49,000 and after $44,000.

Travel time costs were reduced from 13.8 to 10.3 cents/VKT (22.2 to 16.6 cents/VMT) and operational costs reduced from 4.5 to 3.8 cents VKT (7.3 to 6.1 cents/VMT). Total direct operating costs were reduced from 57.1 to 18.7 cents/VKT (92 to 30.1 cents/VMT).

Winter maintenance resulted in direct user benefits greater than the direct maintenance cost:

* During the first four hours after zero hour, the direct road user benefits were $6.50 for every $1.00 spent on direct maintenance costs for the deicing operation.

* The average direct costs are offset by direct benefits as soon as 71 vehicles have driven over the highway.

* The winter road maintenance service pays for itself within the first 25 minutes of the first one hour after the "zero" hour of salt spreading.

Multi-Lane Divided Freeways. Winter maintenance using salt or salt and Ca|Cl.sub.2~ only as the deicer reduced accidents as follows (comparing only 2 hours before and after).

* The rate for all traffic accidents before salt spreading is about 4.5 times higher than that after (3.40 accidents/MVKT before and 0.75 accidents/MVKT after).

* The rate for injury traffic accidents is about seven times higher than that after (2.55 injury accidents/MVKT before and 0.38 injury accidents/MVKT after).

* The rate for property damage (PD) traffic accidents is about two times higher than that after (0.85 PD accidents/MVKT before to 0.37 PD accidents/MVKT after).

The severity of traffic accidents is reduced. The ratio of injury to property damage accident rates before (2.55/0.85 or 3.0) is about 200 percent higher than the same ratio after (0.38/0.37 or 1.0).

Winter maintenance reduced traffic accident costs from "before" to "after" by 85 percent and reduced the average cost of an accident by 30 percent.

* Traffic accident costs rate before was $196,273/MVKT ($316,000/MVMT) or 19.6 cents/VKT (31.6 cents/VMT) and the cost rate after was $30,435/MVKT ($49,000/MVMT) or 3.0 cents/VKT (4.8 cents/VMT).

* The average cost of an accident before was $57,500 and after $45,500.

Travel time costs were reduced from 8.3 to 6.9 cents/VKT (13.3 to 11.1 cents/VMT) and operational costs reduced from 5.3 to 4.8 cents/VKT (8.6 to 7.8 cents/VMT). Total direct operating costs were reduced from 33.2 to 14.7 cents/VKT (53.5 to 23.8 cents/VMT).

Winter maintenance resulted in direct user benefits greater than the direct maintenance cost.

* During the first two hours after zero hour, the direct road user benefits were $2.00 for every $1.00 spent on direct maintenance costs.

* The average direct costs are offset by direct benefits as soon as 280 vehicles have driven over the highway.

* The winter road maintenance service pays for itself within the first 35 minutes of the first one hour after "zero hour" of salt spreading.

Inconclusive results from salt and abrasive mixtures were obtained due to limited experience and are not reported. A major effort on the scale of this test for abrasive/salt mixtures was recommended for further research and is currently underway at Marquette University. Research on the indirect benefits of winter maintenance is noticeably lacking and should be addressed with a substantial study like that of the indirect costs of ice control reported by the Transportation Research Board.
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Author:Kuemmel, David A.; Hanbali, Rashad M.
Publication:Public Works
Date:Jul 1, 1993
Words:2247
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