Understanding driver location signs on the Motorway

There's been a lot of funny blue signs cropping up lately - about every 500 metres in both directions along many English trunk roads. They are part of a Highways Agency scheme, trialled at first on the M6 in Cheshire, then the M25's southern section and the M42 east of Birmingham. Now they're gone national.

 

How do they work?

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The text on the new signs provides all the information the emergency, highway or breakdown services would require to pinpoint a motorist's exact location.

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M27
The number of the road you're travelling on.

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B
The carriageway — A is usually the side on which junction numbers count up, B is usually where they count down. For example motorways radiating from London, A is 'Away' and B is 'Back' again. Other letters are used for slip roads — J, K, L and M are frequently used for simple interchanges with four slips.

 

2.8
The mileage (in kilometres) from the notional start point of the road. The signs appear every 500m, so numbers count up 300.0, 300.5, 301.0, 301.5 and so on. There are some sections of motorways where something is in the way of signs and it is easier to post them at .3 and .8 than .0 and .5.