An evaluation system for special-purpose vehicles (SPV) passage on highway bridges was improved in order to address the commonly overlooked scenario of parallel traffic flow between SPV and regular vehicles during bridge load assessments. The response analysis and multi-level safety evaluation of SPV crossing bridge under unclosed traffic conditions were conducted. Statistical analysis was conducted on the load data of SPV (including weight and dimension information) from a specific province, leading to the identification of representative vehicle types. The composition of bridge loads for SPVs during non-closure traffic was determined, and a four-level safety assessment methodology was developed. The effects of parallel traffic by SPVs on bridge loads were analyzed by using 17 typical medium and small span bridges (bi-directional and four lane) as examples, and axle number limits for parallel traffic at different levels were proposed. The research findings show that SPVs can be classified into flatbed and concave types, further divided into five structural types and three calculation models based on axle spacing distribution and loading patterns. The effects of parallel traffic by SPVs gradually decrease as the safety assessment level decreases, allowing for an increased number of vehicles to travel in parallel while requiring enhanced safety measures. The maximum load effect on the main beam near the center of the bridge deck should receive particular attention during parallel traffic by SPVs. Only a limited number of low-quality oversized vehicles can travel in parallel on RC hollow slab bridges with spans less than 10 m, while PC beam bridges with a condition rating of class II or above can meet the majority of requirements for parallel traffic by oversized vehicles.