Strange objects rather resembling storm lanterns dangling from the roof at intervals through the tunnel have recently appeared.

Anyone have any idea what they are?

Sorry if this is off topic, but it is undeground.

Do they light up?

Got a feeling its part of a new public address system.
Saw signs for this being tested.

Not to my knowledge. They only vaguely resemble storm lanterns. The place where the glass might go if they were storm lanterns appears open.

Think of a square plate about 9" by 9", with a similar plate about 9" below it held apart by four bars in the corners. Below the lower plate is a cylindrical assembly.

Apologies if the dimensions are a bit off, but I was driving at the time and I think they cops might have objected to my stopping for a photo.

I thought they were something to do with monitoring the amount of fumes in the tunnel

This could be a possibility, MrHouston. Just dust collectors perhaps?

I was unaware of any plans for a public address system, but they are a long distance apart for that. I didn't count but I'd reckon perhaps ten distributed through the length of the tunnel. I left via the dock exit in Liverpool and there were one or two there too.

Probably state-of-the-art Automatic Incident Detection System.

http://www.clickliverpool.com/news/...n-system-within-the-mersey-tunnels-.html

If you check out pictures for "Hindhead Tunnel" you will see a very similar object.

Yes, as OxtonHill says. They are radar scanners - like this:

Yes. That's the animal all right. In the tunnel the two plates are further apart, and it is upside down, but its clearly the same device otherwise.

Problem solved. Thanks.

Speed catches chaps

Mmmm.... I see there is a metal priming tray holder underneath the unit. The unit appears to be made of some sort of plastic/polypropylene - which could melt or catch fire if a shallow tray of flaming hydrocarbons are placed below.

Merely an observation of course..........

DON'T TRY THIS AT HOME KIDDIES !!

I doubt if they'd be any good as speed traps. You have to identify the speeding vehicle as well as detect it is going too fast.

I think they are intended only to sound the alarm if traffic stops.