imperial - they are reflective. I have no idea why they are white on the front and yellow on the back, but people often park on the wrong side of the road, legally would you believe, so that and fog could well be the reason. Also when I was there, they would drive around at night with only their parking lights on, so in the dim foggy damp conditions, it may have helped. My 100E Anglia was black, so the first thing I did was put reflective plates on so I could be seen. These pics were taken at about 3:00 pm on a winter's day.....