[Closed] Road markings - the small white rectangle in the middle of a lane, what's it for

Hello all,

Sometimes in the middle of a lane there will be a white rectangle, just less then 2 foot wide and 1 foot high. Anyone know what they are for? I ride/drive over them often enough but have no idea. Odd thing to ask I know.

The closest I have come up with is a police designated speed test area, however that is a square and not a rectangle. I can't find it listed.

[url= http://www.dft.gov.uk/pgr/roads/tss/tsmanual/trafficsignsmanualchapter5.pdf ]DFT Traffice Sign manual chapter 5[/url]