What are the black boxes on top of trafflic lights? (2024)

Hi, What are the black box's that sit on top of traffic lights not all have them but alot with crossings do.

I have been told they are sensors for the lights to change or something not sure if this is right?.

Yes, they detect vehicles and pedestrians.

The one facing down towards the pavement detects if a pedestrian walks away after pressing the button, and cancels the lights changing. The one facing sideways onto the crossing detects a pedestrian's progress across the road, and holds the lights at red (within reason) until they make it to the other side.

As for the ones facing cars, at pedestrian crossings they monitor for gaps in traffic so the lights change to red when oncoming traffic is less busy, avoiding holding up too much traffic. At junctions, they detect a vehicle arriving at a red light and register the need for those lights to change to green.

and in case anyone ever tells you that flashing your headlights will fool the sensors into thinking that you're an emergency services vehicle and will change the lights to green, you can tell them they're wrong.

I believe the sensors on top of the signals work using Microwaves and simply respond to the presence of any vehicle or person (depending on how they're configured).

Mark Hewitt wrote:If you look carefully (most/all?) will have a red LED light on them to indicate they have been activated.

Yes, I've noticed that.

Railway, road, sky, football, rugby enthusiast since 2001

Proud to share a birthday with the M25!!

The one facing down towards the pavement detects if a pedestrian walks away after pressing the button, and cancels the lights changing. The one facing sideways onto the crossing detects a pedestrian's progress across the road, and holds the lights at red (within reason) until they make it to the other side.

That would explain a lot! A couple of years ago my daily commute involved crossing the A4 at the lights on the Jacobs Wells roundabout in Bristol (link: http://goo.gl/maps/fAlbS) and even though I would press and stand there patiently during the first few weeks, half the time the lights just ignored me - the red ring lit up to show that the button had been pressed, only to go out again afterwards without my light changing to green. I guess the sensors were either badly calibrated or picked up traffic passing on the other side.

I have to cross https://www.google.co.uk/maps/@51.56612 ... hcnvKg!2e0 daily after getting off the bus and I have the same problem. If you dare to move an inch the light for the crossing will go out.

Railway, road, sky, football, rugby enthusiast since 2001

Proud to share a birthday with the M25!!

RJDG14

See my Geograph profile here - http://www.geograph.org.uk/profile/74193
The Swindon Files - Swindon's modern history - http://rjdg14.altervista.org/swindon/

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If I break a policy designed only to protect me and nobody else, have I really broken anything?

These types of detectors are referred to as AGDs which is an initialism of Above Ground Detector. Previous detectors were only either loops in the carriageway (look for lozenge or rectangular lines of bitumen in the carriageway ahead of the stop line) and pedestrian push-buttons which are still used today. Simpler junctions without crossings may have and in some cases continue to run Fixed Time (FT) whereby they simply cycle regardless of the queues thay may or may not exist on any or all approaches to the junction.

Most modern temporary traffic signals have AGDs and are radio-linked. If the radio link drops out that return to fixed time.

The AGDs detecting on-coming vehicles are usually MVDs (Mircowave Vehicle Detectors), however there are also IRDs (Infra-Red Detectors) used to determine waiting vehicles at the stop line.

IRDs also 'look' across the crossing are esentially black and white cameras. On some, if not all models the engineer uses a computer plugged into the traffic signal controller (TSCU) and draws the outline of the tactile paving so the IRD has an image of the empty crossing which it compares with whatever the existing image is to determine if someone is still waiting once the button has been pressed.

The bleeper (audible) is in the push button uni (PBU) - when there's crossing near to each other than don't run at the same time the audible is replaced with a tactile which is a small cone on the bottom of the PBU which rotates.

I once took a call from a gentleman complaining the audibles were no longer working and explained the site was equipped with tactiles and what they were, he asked "Do blind people know this?!" I responded "I'm sure they do."

Twamgle wrote:These types of detectors are referred to as AGDs which is an initialism of Above Ground Detector. Previous detectors were only either loops in the carriageway (look for lozenge or rectangular lines of bitumen in the carriageway ahead of the stop line) and pedestrian push-buttons which are still used today.

And before loops there were pneumatic strips embedded in the road, like these.

Chris
Roads.org.uk