The snowy midwestern US state of Ohio wants fewer crashes with their snow plowing and road salting trucks, so that state’s Department of Transportation is trying a new idea: flashing green lights in addition to the standard flashing yellow ones. This effort makes Ohio one of the first US states to add a second colour of lights to their snow trucks, according to ODOT spokesman David Rose, and the first ever to use green.
Rose says the impetus is roadway safety, particularly in view of the large speed differential between snow trucks and other vehicles. He says drivers overestimate the plow trucks’ speed and come up on it quickly because the trucks, when on duty, “travel 35 miles an hour (55 km/h) or less, and on interstates (motorways) that causes a safety issue for drivers…there’s a depth perception that makes it hard to know the vehicle in front is going much slower.” Rose says the second colour of flashing light will improve drivers’ ability to correctly assess the plow trucks’ speed and distance.
In 2009 there were 57 crashes wherein vehicles hit snow plow trucks. In 2010, there were more than 60. Drivers attempting to pass snow plows create an additional hazard, because the plow trucks create a snow cloud that can suddenly drop visibility distance to almost zero for the driver who enters it. Drivers may attempt to pass snow plow trucks because they cannot immediately distinguish them from other vehicles that use amber lights, such as tow trucks, delivery vans and construction vehicles.
For ODOT’s Rose, green makes sense: “It just so happens our logo is green and it just seems to be a natural fit,” he says. “Our equipment guys have been studying this for over a year. Green is one of the most visible lights to the human eye.”
DVN Analysis
This is an interesting development. On North American roads, green warning lights have long been more or less officially reserved for volunteer firefighters and other similar ancillary emergency services. For that reason, most drivers only very infrequently encounter green warning lights, so that colour is not overloaded with numerous possible meanings. And as ODOT’s study has determined, green is at the peak of the human visual system’s sensitivity curve. Moreover, there is research supporting the notion that green lights convey particularly accurate distance and position information in the nighttime driving environment—considerably more so than other colours such as red or blue. Such is the degree of this difference that a 1964 paper decried the crash-inducing effect of the “misuse” of red for the tail (rear position) light function, and called for that function to be made green instead. It is also encouraging to see that ODOT has chosen LEDs rather than Xenon flashers for the new green lights. Nevertheless, it appears from the publicity photo that the agency might have missed an opportunity to optimise the crash-avoidance potential of the new lights. Recent research (such as that from John Bullough and associates at RPI-LRC, in the equally-snowy state of New York) shows a positional-conspicuity benefit to vertical light arrays at the left and right of the rear of snow removal equipment, rather than the top and side mounted flashers Ohio has chosen. Still, colour conventions are often a matter of long running tradition, and they tend to change only very slowly and by dint of outside-the-box thinking.it will be interesting to keep an eye on the results from this practical experiment.
