One of the solutions to improve the eco-efficiency of road freight transport is to combine existing transport modules into Longer and/or Heavier Vehicles (LHVs). The scientific and industrial communities have paid increasing attention to an LHV, known as the A-double combination, consisting of a tractor, two 13.5-m semitrailers, and a dolly converter. The present research contributes to the existing literature by developing a methodology based on a cost-benefit approach to quantify the effects of the A-double combinations if introduced to a line-haul transport system. Four implementation scenarios and sensitivity analyses of main variables were evaluated within a case study of 48,472 line-haul trips in Denmark. The results indicate that in the least beneficial scenario, the A-double combinations reduce transport cost by 9.65% while reducing trips, CO 2 emissions, and road wear by 17.91%, 5.34%, and 9.55%, respectively. Besides, the use of A-double combinations can significantly reduce empty tractor-semitrailer trips. However, the benefits are relatively less in the case of just-in-time deliveries and cargo constrained by vehicle weight. Also, cost saving is highly sensitive to driver salaries, fuel prices, and driving speeds. This research provides valuable insights into the potentials of A-double combinations under different regulations and freight characteristics from a micro perspective.