Motoway objects

[Beswad]

It's been bugging me for a long time I would like to know what are those dark blue rectangle boxes that hangs on every bridge on the motorway facing on coming cars?

They can't be number plate recognition or speed cameras because they were there long before speed camera were introduce. They look like cameras but show no obvious sign of any camera lense. Are they sensors for traffic control or pollution monitors?

Is there a site which explains every objects install in our motorways?

[Impret-Sir]

They are part of a network that monitor traffic congestion and allow drivers to be warned about bottlenecks etc, if you have the right little box in your car!

[sddsddean]

You also see them on poles at regular intervals on most A roads.

[Octavius]

They measure the rate/speed of traffic to send the signal to the AA or whoever to give information on traffice flow levels.  This info is then used for satellite navigation and traffic jam reporting.

[CliveB]

Called traffic master.