Non Public Road Signage

[Plotloss]

Curiosity as now got the better of me and I have to ask if anyone knows.

On the M4 heading into town there is a blue sign with white and yellow lettering just before Heston.

It says:

M4
021 B

To what does this refer?

No real reason to know other than the aforementioned curiosity...

[chrishillcoat]

It's the distance from Charing Cross (in km), and "B" is the direction you're heading. Spot them on the M25 and M6 as well