Driving along the road, coming up to a set of traffic lights, for example, have you ever noticed black lines, forming squares or rectangles in the tarmac (circled in red in the above picture) and wondered what they were?
They are called Detector Loops and understanding how they work will help you anticipate whether a traffic light you’re approaching is likey to change against you or help you to activate a filter light if you intend to turn right. They are often found on busy main roads and at junctions with housing estates etc.
After the tarmac is laid, at traffic light controlled junctions, the contractor sawcuts the road surface and installs coils of wire into the groove. The sawcut is then filled with a rubber mastic compound to seal the road surface. These wires are laid under the road surface back to the junction control box which changes the sequence of the traffic lights.
Looking at the above picture, the road running bottom to top is the main road and therefore has priority for traffic flow. Once traffic is freely moving through this junction bottom to top or top to bottom the vehicles will pass over the detector loops (circled in red) sending signals to the control box which keeps the main road lights at green. If traffic moves onto the detector loops on either of the two side roads they too send a signal to the control box saying there’s traffic now waiting. If there is a sufficient gap in traffic on the main road, i.e. no vehicle is moving across the detector loops, the lights on the main road will turn red, allowing the traffic on the side road(s) their chance to go. If no traffic moves onto the detector loops on the right or left the light may never turn red on the main road. This knowledge can now help you to anticipate the chance of a traffic light turning red as you approach. In other words, as you approach the light look to the side roads (if visible) to see if there’s traffic waiting. If so, there’s a possibility the light might turn red against you. If there’s a big enough gap between you and the vehicle in front this possibility is increased. The light could also turn red against you anyway as the light sequences are normally on a timer which can be overridden depending on traffic flow.
Turning Right
Look at the car circled in blue, it is waiting to turn right.
When the light goes green this car should move forward of the stop line and sit in the turning box. This box also contains a detector loop (red arrow pointing to it). However, if the car stays where it is in the picture the control box will not know that someone is sitting in the junction waiting to turn right. This means the the right hand filter light will not come on and the car could miss their turn at the lights. This is something I see on the roads day in, day out. Not moving into the box affects traffic flow and could, if you were sitting your driving test, result in you failing.
This post is for information only. This information does not reduce your responsibility to assess the situation or road/traffic conditions in front of you as you approach junctions.
