Why is red light on speedometer coming on?

I have an old ford cargo horse box. On the speedometer there are two red lights. one on the bottom left next to an exclamation mark in a triangle. the other is on the right of the speedometer and comes on when I get to 50mph, that I understand. The light on the left, however, has recently started coming on about 10 mins into my journey and stays on for about 5 mins, then flickers for a bit, then goes off sometimes coming on again later. I am just wondering what this red light is telling me? It has only started doing this in the last 3 or 4 months!