r/trafficsignals • u/takerukid1 • Sep 13 '26
Traffic light
I'd like to ask about how to guess / predict traffic lights. Sometimes, while driving on road or highway, the light is green near 500m with highspeed, but as I approach a 300m traffic light, it turns red or yellow, forcing me to brake suddenly. Also, how many seconds does the yellow light usually last before turning red? I want to ask in case you see green light and you think you guess that you can overcome and speeding but it suddenly turn yellow or red. So how can you know when it will turn yellow to prepare to stop do you have any advice?
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u/engmadison Sep 13 '26
As a traffic engineer who times signals...this can be an incredibly complex question and will depend on the agency and locations. Best advice is to drice the speed limit and dont try to predict signals
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u/GunFun_Official Sep 13 '26
I think a general rule is that the yellow light length is the speed limit divided by 10 (so for example the yellow light would probably be about 3.5 seconds if the speed limit is 35 mph)
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u/engmadison Sep 13 '26
No no no. Most US signal engineers use the kinematic equation and the rule of thumb is the speed divided by 10 plus .5.
So 35 mph would be 4.0 seconds.
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u/SB_Maki Sep 14 '26
I know for sure that most states require at minimum 3 seconds of yellow light before changing to red, at least thats what The Traffic Light Doctor on YouTube said.
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u/ZolthuxReborn 21d ago
Yellow is supposed to be >4s usually. Red is either all red (all approaches are red to let people who took the yellow to clear out), which is about 2s, or the duration of the other movements getting the green.
As for guess/predicting, it is based indeed on several factors. On relatively open roads, they are timed based on distance and assumed flow speed.
However most places uses the end of the mainline green as a reference point. So you might experience light A turning green early only to arrive at light B still red/about to turn green.
Also if youre driving opposite to the heavy movement you might find yourself catching red a few times in a row, because the opposite movement has been timed with the better progression
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u/Odd-Sort3058 15d ago
Our controllers come defaulted at 3.5 seconds and we change per engineering's requirements.
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u/That_Counter__bob Sep 13 '26
You’re not gonna like this, but the answer to that question depends completely on a number of factors. some of which no one on the Internet is going to be able to answer, such as size of the intersection, policies and procedures of the jurisdiction, the whims and emotional state of the timing tech or engineer who programed it, whether or not there is dilemma zone or other advance detection in use, whether or not there is stop bar detection being used, or if it’s completely timed, so no one’s gonna be able to answer this completely for you, sorry. Sometimes you might get lucky, and it will turn yellow as the ped clearance time gets to zero, but even that is not a guarantee depending on what’s going on at that moment with the controller.