Nowadays, there is no exact method for determining the optimal speed limit in winter and the problems associated with winter speed limit has not yet been addressed in Hokkaido. Therefore, this study aims at determining the optimal winter speed limit through the application of a cost analysis and by analysis of effects of road and traffic conditions. Initially, a cost analysis of travel time costs, vehicle operating costs, pollution costs, and cost of accidents was applied to determine optimal average speed on the basis of the minimum total cost. Then, the effects of road and traffic conditions were calculated by regression analysis. Finally, the optimal winter speed limits were achieved by adding those effects to the optimal average speed. For the purpose of reliability, we applied a sensitivity analysis to the model. We found that our model was reliable and the results are appropriate and sustainable long term.