I think riding during the winter months is only more dangerous if you don't adjust your riding style accordingly. I ride year round and have for the last several years. I've many more close calls in the summer months than the winter, mostly due to the fact that I ride more aggressively in the summer. Adjusting my riding for the different conditions of the roads and the way my tires are going to react, as well as the fact that cars may not be on the lookout for bikes as much has kept me just as safe in the winter as summer. So basically I think it's up to the rider to be aware of different circumstances and like I previously stated, adjust his/her riding accordingly.