"Have seen the van at J15 M4 a few times after it has rained and a bit of dlzizre is still about. Road spray could possibly affect the result as the water droplets will contain material that could affect the speed of the returned signal, and indicate a greater road speed they do not work 9 -5 seen them at 0800hrs and in the evening before it gets dark maybe as late as 2100hrs, cant say I have seen them in the dark though could be the pubs are open then!"