If a vehicle wheel passes over a road surface irregularity it will be excited into vertical oscillatory motion and will generate dynamic variations of wheel load about its static load. A test lorry, instrumented to measure dynamic variations of the load on its rear offside wheel, has been used to assess the variations that occur in practice on highway bridges. Measurements on thirty motorway and over-motorway bridges showed that on average the motorway bridges had smoother surfaces; they had peak values of impact factor (ratio of dynamic to static wheel load) of from 1.09 to 1.43 while the overbridges had values of from 1.16 to 1.75. Most of the peak values occured near the start of the end of the bridges. One bridge carrying an unclassified road over a motorway was found to have an abrupt step, about 25 to 31 mm high, in the running surface between the embankment and the bridge, and this gave a peak impact factor of 2.77, much the largest measured in the series of tests. Two long span bridges and three multi-span viaducts were also examined and these had peak values of from 1.25 to 1.66. The effect of vehicle speed and of tyre type was also examined. The significance of the results in relation to the design allowances for impact factor is discussed.