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Traffic Lights And Sensors


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Traffic Lights And Sensors
Controls For Traffic Lights

Traffic Lights At Junctions

Traffic light timers

Before discussing traffic light sensors, it helps to know about traffic light timers.

Traffic lights are normally controlled by timers. These timers change the traffic lights at set times during standard traffic flow conditions. At a crossroad junction for example, the timing could be set for the main road to have 30 seconds of green, then the side road to have 20 seconds of green, then back to the main road, and so on. At certain times of the day, and usually during certain times of the night, the timers are changed to different times. As an example, during certain night time hours the side road at a junction might be set for the traffic light to remain on red all night except for when a vehicle approaches, and that's one example of where the traffic light sensor comes in (explained later in this page).

Lauren Reid of Irvine, Driving Lesson Testimonial
I was so nervous when it came to my first ever driving lesson, Dave made me feel at ease and he helped me through every step of the way, he was so calm and helpful. His patience helped me a lot too as my nerves got the better of me at times. I highly recommend Dave to anyone as I passed my test first time on Friday.

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Traffic light sensors

Traffic lights at junctions usually have sensors that assist with the flow of traffic. These sensors are usually located:-

  1. directly above the traffic lights;
  2. or, buried within the road on approach to the traffic lights and/or where the first vehicle would normally wait if the traffic light is on red;
  3. or, a combination of both sensor types.
Ross McDowall of Dreghorn, Driving Lesson Testimonial
Dave is a great driving instructor whose teaching prowess enabled me to pass first time with no minors. He was flexible with very short notice booking and always ready to accommodate. He's a patient and thorough teacher, and his stressless teaching style really helps in getting comfortable on the road. Highly recommend!

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The sensors normally located directly above the traffic lights

These sensors detect the body of a vehicle approaching the traffic lights, on some junctions the traffic lights will not change to green until a vehicle is detected approaching them (they cannot detect light, so for those who flash their headlights at roadworks traffic lights to encourage them to change, it does not work, if light did cause the traffic lights to change then the traffic lights would be constantly changing during daylight hours, causing mayhem). Examples of how these sensors control the timing of the traffic lights are as follows.

  1. When a vehicle approaches, the traffic lights will either change to green on approach or change to green at the next timed allocation;
  2. The traffic lights will remain on red until a vehicle approaches;
  3. A few more seconds will be added to the green if a higher volume of traffic is detected to be approaching;
  4. When the green comes on, time will be deducted from the green (this could mean down to as little as a couple of seconds) if a lower volume or no traffic is approaching.
The timer settings and the sensor controls for each junction are set according to the traffic studied at that junction and the layout of the junction.

Jack Keenan of Irvine, Driving Lesson Testimonial
Would just like to thank David for helping me get a first time pass today 29.10.21. I couldn't of done it without him what an amazing instructor. Very patient and very calm demeanour. His teaching style allowed me to feel stress free while doing my lessons and he was very flexible when booking the lessons, especially on short notice as well might I add, He also made sure there was plenty of room for my lessons the week before my test so I could get the slots I wanted. Thanks again David Jack Keenan

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Unfortunately there is an issue with this type of sensor when there is a traffic queue waiting while the traffic light is on red. The issue arises if the first vehicle in the queue attempts to move off when the green comes on but they stall. Some traffic light systems are programmed to almost immediately change back to red if there is no traffic detected to be moving. Now the real issue comes in. As mentioned earlier, some systems are programmed to remain on red until a vehicle approaches. Due to the traffic being queued up at the time the traffic light changed to red, there is no traffic moving towards the traffic light, or the sensor, therefore on some systems the traffic light will remain on red and not change back to green. A very difficult situation to be in.

A similar issue arises when the traffic moves off on the green but a vehicle in the queue takes time to move off, thus leaving a longer than normal gap between that vehicle and the one in front. The sensor detects the gap as an indication that no more traffic is approaching and thus changes the traffic light back to red. This will mean fewer vehicles being able to continue and they will have to wait until the next green.

Danielle Bennison of Bourtreehill, Driving Lesson Testimonial
Can't recommend Dave enough! His approach to teaching is calm, patient and at your own pace. Thanks to Dave, I passed first time!

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Elisha Bennison of Bourtreehill, Driving Lesson Testimonial
Before I started my lessons, I was very anxious on the road with other drivers due to being a passenger in a crash and thought I would be a nightmare to teach. Right from the start Dave was always patient, calm and understanding. He would walk me through each point to improve and was very knowledgeable about the road and how to prepare for my test. I'm happy to say I passed my driving test first time and it wouldn't have been possible without Dave as my instructor.

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Karen Bennison of Bourtreehill, Driving Lesson Testimonial
I highly recommend Dave as an instructor, he was a lovely guy. He is patient and calm, which I definitely needed. He made me feel at ease and this contributed to me passing my test. Thanks again Dave!

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The sensors buried into the road

These sensors can usually be seen in the road as a line creating an approximate one metre square or as lines looking like a large hash marking. These sensors detect pressure i.e. they detect when a vehicle is on them.

These sensors change the traffic lights just like the sensors above the traffic lights, but with a few added features and a significant difference. Examples of the extra features are as follows.

  1. At some junctions, there is a sensor buried in the road out at the point where a vehicle would wait for a gap in the traffic before turning right. This sensor is usually for controlling the filter arrow. Examples of theses controls are:-
  2. If there is a queue waiting to turn right, then the green arrow will come on allowing the traffic to turn;
  3. If there is a long queue waiting to turn right, then a few seconds will be added to the green arrow, allowing more vehicles to turn;
  4. If only a couple of vehicles are waiting to turn right (and they are out in the middle of the junction waiting to turn), then sometimes the green arrow is not turned on. This is because the couple of vehicles should turn right as soon as they have a gap in the traffic, or turn right when a gap is created by the traffic light turning to red for the oncoming traffic. This allows for better traffic flow.
  5. At some junctions, traffic can only turn right if the green arrow comes on (there is no green circle). If a sensor is buried in the road just before the wait line at the traffic lights then some green arrows will not come on until a vehicle has driven onto the sensor. So if you approach and the traffic light is on red, make sure your vehicle is on the sensor or you'll be waiting there all day.
Charlie Wilson of Irvine, Driving Lesson Testimonial
No need to look any further. Davy will be the instructor for you. Whether you clip a kerb at 40 miles an hour at the beginning of your lessons (sorry!), or argue with him towards the end because you think you know better (I didn't and again, sorry!), Davy handles it all with an abundance of understanding and patience. His depth of knowledge is second to none! He adapted his teaching style to suit my needs and was completely flexible about working around my schedule. I guarantee that when your test time comes you will have all the tools, skills and confidence to pass first time. I did - and it was all thanks to Davy.

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So what's the significant difference with these sensors buried within the road. These sensors have the same issue for when the first vehicle stalls when attempting to move off. The difference though is that when the traffic light changes back to red, the system will recheck the pressure sensor and if a vehicle is positioned on the sensor then the traffic light will change at the next designated time.

At some traffic light junctions, both types of sensor are used in combination to assist with the control of traffic flow.

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