Impact of Weather on Freeway Capacity

Roadway geometries, vehicle type and traffic flow characteristics are factors commonly referred to in discussions of freeway capacity, primarily because they are significant and can be influenced by the application of traffic engineering techniques. There is another factor, or rather group of related factors, that can be classified under the broad term "weather," which have often been ignored in the sense of quantitative studies, although traffic engineers and motorists alike have cursed and praised many a day's weather. One reason weather factors have been largely neglected is chat they are, unlike geometries, not directly in the control of engineers. They also are discontinuous and therefore difficult to plan on or respond to - after all, if there was always two inches of snow on the pavement, or if it continually rained, we could make adjustments. As it is, one common reaction to bad weather is simple despair.

Recently, attention has been paid to flexible traffic management techniques which, although still not putting engineers in control of the weather, allow management personnel to at least positively respond to weather conditions. An example of such flexible traffic management is the use of a metered freeway system, such as that along I-35W south of Minneapolis in Minnesota. Although it cannot hold off a snowstorm, such a system allows the input to the freeway to be controlled so that the reduced capacity is not exceeded and the accidents connected with bad weather are avoided. The question which management personnel must answer to respond appropriately to a particular weather situation is, "given this much rain (or snow or wind) how much will traffic flow be affected; that is, how severe should my response be?" Unless system operators know to what extent a particular situation threatens to reduce traffic flow, they can not respond confidently and appropriately. Because flexible traffic management is becoming increasingly common as a way to maximize the use of highway facilities when energy costs are high and construction dollars dwindling, it is time to take a quantitative look at the impact of weather on freeway capacity.