Abstract: The concept of visibility distance of the traffic signs was proposed and the constraint conditions were established based on the analysis of the process of using traffic signs to ensure the driver to have a sufficient time to recognize the information after detection of the traffic signs in the complicated environment.In order to further guarantee the possibility of the vehicle to change the driving condition safely and smoothly before a hazard point or a road intersection,a model for the placement of the roadside traffic signs was built based on the constraint conditions for the visibility distance of the traffic signs and the analysis of the problems existed in the present placement method of the traffic signs with consideration of the effect of the traffic sign placement on the roadside safety.

Key words: traffic safety, traffic sign placement, roadside safety, visibility distance