The index equalled 112.5 for the month, up 2.2 per cent from a reading of 110.1 in March and 9.5 per cent compared with April 2017, the largest year-over-year increase since October 2017.Through the first four months of the year, domestic truck volumes have grown 8 per cent compared with the same 2017 period, more than double the annual growth rate of 3.8 per cent seen last year, reports American Shipper.The association's not seasonally adjusted index, which represents the change in tonnage actually hauled by the fleets before any seasonal adjustment, equalled 109.2 in April, down 4.8 pe recent from the previous month's reading of 114.7."Truck freight tonnage remains robust. And I don't think we've even seen the traditional spring freight season yet," ATA chief economist Bob Costello said in a statement. "People are just getting around to buying grills, lawn mowers and yard tools. Plus, the produce season was also delayed due to a cold snap in early spring."Longer term, strength in consumption, factory output and construction should keep truck freight tonnage solid for the quarters ahead," he added.According to analysts the continued growth in demand is likely to exacerbate a US trucking market that is already tighter than it has been since the 2004 capacity crunch, putting further upward pressure on rates. Domestic truckload rates, for example, have now increased in each of the last 13 months on a year-over-year basis and show no signs of slowing anytime soon.