Subtle and intermittent icing (not visibly prominent)
Light winter precipitation (including snow and freezing rain)
Freezing rain/drizzle/fog (invisible ice)
In essence, the road ice hazard is primarily highway-speed travel during light winter precipitation events, when driver awareness is low and visual indicators are few. The majority of deaths and serious injuries occur during these conditions. To narrow the definition of the hazard even further, we can cite the phenomenon of high-speed oversteer. Oversteer is the primary cause of loss of vehicle control at high speeds, and most fatal accidents begin with one or more vehicles entering an oversteer condition. Oversteer at highway speeds is nearly impossible to correct and leads to a vehicle spinning out, leaving the roadway, rolling over and/or impacting oncoming traffic.
It might be helpful to list what the road ice hazard is not:
Snowstorms: Fatal and injury accident rates are very low during heavy snow events, mainly due to the fact that the hazard is plainly apparent to motorists. Awareness levels are at their highest during heavy snow. Many people postpone or cancel travel and commuting during snowstorms. The reduced visibility and snow-covered roads make it harder for drivers to attain unsafe speeds, and most incidents that do occur are minor. The highest crash rates during a heavy snow event occur at the very start of the event and on its fringes, where snowfall is light.
Snowpacked roads: Fatal and injury accident rates are very low when roads are completely snowpacked during and after a significant snowfall. Again, this is attributable to the high visibility of the conditions and their inherent limitation on attainable vehicle speeds.
Urban hills: The popular and frequently-televised 'viral' videos of multiple cars sliding and crashing down icy hills in cities and residential areas is also not truly representative of the road ice hazard. These incidents typically involve slow speeds and minor impact forces. While dramatic and certainly damaging to vehicles, this type of event rarely results in injury or loss of life. The greatest risk during these incidents seems to involve the 'secondary crash' phenomenon, where motorists exit their vehicles and stand in the roadway, directly in the path of additional out-of-control cars and trucks. I have not documented a single fatality from this incident category during the two years of data from 2008 to 2010.
Any public awareness value of these types of popluar videos seem to be lost by their portrayal as humorous in media and internet discussions.
To sum it up, the road ice hazard isn't the minor fender-benders or slide-offs common during snowstorms. The hazard is primarily the serious, highway-speed crashes during light icing events that take drivers by surprise.