Allocating and routing problems in the field of transportation engineering are generally solved using the objective function of minimizing transport cost. Transport risk is a main concern in hazardous material transportation, mainly dependent on the vision of decision makers (national and/or local authorities). In this research work, a trade-off approach has been proposed to determine the safest paths and the best locations of distribution centers for carrying the third type of hazardous materials categorized as flammable liquids. Trade-off has been defined between risk and cost, whereas a bi-level objective function has been developed to determine the best routes for hazardous material transportation and also to determine the best locations for establishing the main distribution centers. At the first level, obtaining the best locations for distribution centers are obtained then determining the safest paths of origin-destination is developed at the second level. Experimental data in the second largest Iranian province of Fars which consists of fifty-nine nodes and eighty edges has been used for applying analytical process. Results show that decision makers should be aware in the process of allocating and routing problems as well as influencing attributes before to make decisions, because the priorities of risk and time and the number of distribution centers play significant roles in routing and allocating hazardous materials.