An important issue in Hazardous Material (hazmat) transportation risk assessment is to evaluate the probability bounds of accidents occurrence, whose values are difficult to be estimated due to its low frequency and the related lack of statistical data. This paper presents an original approach to integrate uncertainty in the quantitative analysis of hazmat transportation accidents. The proposed approach is based on the use of Valuation-Based Systems (VBSs) and belief functions theory. Furthermore, we propose to identify the factors for which the reduction of epistemic uncertainty (imprecision) gives the greatest impact on the uncertainty of the final results by using some proposed measures. The applicability and generality of the proposed approach is demonstrated on a case study.