Was there ice underneath or on top of the snow? Most snow plows in urban areas have rubber bits to prevent damaging the road. If there's a thick layer of ice on top of or underneath snow, a rubber bit does absolutely no good. We ran into this problem a few years ago at my workplace when we had freezing rain over a 6 inch base of snow.
Steel bits will cut into the ice but the plow has to be able to have them attached. Of course, if it wasn't for the eco-nazis we could just salt the crap out of the road in a emergency like this and have the roads open in a couple of hours.
I don't understand the salt ban in Colorado. Salt works fine back East. But a side effect is that you have a fair number of fine old classic cars on the road, much more than I see in the Northeast, that haven't rusted out.
A relative of mine drives a plow, so I'm not attacking snowplow drivers, who live a rough life during times like this.