Coming from Canada where speeds are in kilometers an adjustment is needed to mph. After crossing the border and moving at lesser than the posted speed at the border I was checking my speed scale and mentally adjusting myself to the US scale when I missed a posted sign demanding "reduced speeds" and therefore was moving over the speed limit. I was caught at this point going 57 mph in a 40 mph zone. The change of speed zone I think was too close to the border where crossing traffic must make this adjustment.