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CURVE TOO SHARP?

A recent case involved a curved section of a two-lane rural highway under slippery conditions. This section of the highway has been the site of many accidents over the years. Local residents consider this particular curved section of highway to be too sharp.

Situation

Vehicle A was travelling around the inside lane of the curve at a reduced speed because of slippery conditions. Despite its reduced speed, Vehicle A lost control and slid into on-coming traffic, resulting in a head-on collision with Vehicle B.

Investigation

In addition to analyzing speed and reaction times, particular attention was given to the reported sharpness of the highway curve.

Geometric Road Design Assessment

Preliminary analysis of highway geometry confirmed design deficiencies in the curve and transitional spirals (entering and exiting the curve). This immediately implicated the Provincial Ministry, the Municipality, and the design engineering firm.

It was soon discovered, however, that this section of the highway was built in the mid 1970's. At that time, the Standards to which the highway was designed and constructed were still published in imperial units of measurement. During the conversion from imperial to metric in the late 1970's, all numerical design criteria were recalculated for corresponding "round" numbers of measurement (e.g. 60 mph adjusted to 100 km/h). Design criteria were adjusted toward slightly more conservative values, for the purposes of either maintaining or increasing safety factors. Therefore, any existing roads which had been constructed according to the imperial Standards would not necessarily comply with the new metric Standards.

With that information, the design of the highway was reanalyzed with respect to the older imperial design Standards. It was found that all geometric measurements were completely in accordance with the design Standards of the time. Therefore, the various implications of the above-mentioned third parties were dropped. The fact that the curve seemed to be too sharp may have been due to comparisons with newer roadways.

After the Standards had been converted to metric, corrective reconstruction on existing "imperial" roadways was not necessary. Of course, any future modifications such as road widening would be required to meet the newer metric Standards. Corrective reconstruction would be performed at that future time.

Conclusion

The highway was designed and constructed in accordance with the Standards at the time. The focus of the investigation then reverted back to the slippery conditions of the road and the speed of Vehicle A given those conditions.

 
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Walters Forensic Engineering | 277 Wellington Street West, Suite 800 | Toronto, ON M5V 3H2
Information contact: engineering@waltersforensic.com