Two experimental orthotropic deck panels were studied while subjected to normal traffic on a trunk road. The behaviour of 38 mm mastic asphalt and 9mm resin-based surfacings were compared. The stresses generated by moving vehicles were measured and found to be considerably reduced on the panel surfaced with asphalt relative to the panel surfaced with resin-based materials. The composite action of the mastic asphalt with the steel deck panel was investigated over a range of vehicle speeds and surfacing temperatures. Records of vehicle loading and surfacing temperature were made throughout the trials. After four years fatigue cracks developed in three stiffener to cross-beam welds on the panel surfaced with resin-based materials. These were found to be due to stresses generated by traffic and also influenced by an inadequate standard of welding. When the site was closed after more than nine years of traffic loading additional stiffener to cross-beam welds in this panel had cracked. At the end of the trials the mastic asphalt surfacing remained in good condition but the resin-based surfacings had become unserviceable