I have to drive every week to and back from South Devon to Twyford, Berks or one of our other sites that takes me through 3 long sets of SPEC average speed cameras on the M5 and M4. I can't find a good explanation of how one is caught speeding and what tolerance there is at driving at exactly 50mph through such long stretches. I have a 2004 Subaru Legacy 2.5i Sports Tourer with no cruise control. The speedo is also marked at 40-60 etc, so keeping to exactly 50mph is difficult, especially on a downward gradient. Sometimes I find I have crept up to 60 (about 55 on Sat Nav) whilst watching road conditions ahead rather than being glued to my dial.