With heavy traffic in both north/south and east/west directions, what percentage of the time is spent waiting for the light to change?

Impossible to guess; each traffic light is engineered differently by traffic engineers based on time of day, anticipated traffic, whether looking towards the sun, and so on.

They’re set individually.

It changes with the time of day and the suspected traffic load.

If all things were equal you might spend the same amount of time at a red light no matter what direction you are going. However most intersections have a main road and a lesser road meeting. Obviously the light stays green longer for the main drag and less on the side street.

I don’t believe it is fair to say you spend half your time driving at red lights because of this.

There are “pads” under the asphalt at many stop lights. Magnetic I believe. When a car passes over it, something is triggered to tell the light circuitry there is a car waiting. I believe it starts a timer to change the light for that person. And yes, right turns on red are a way to keep traffic moving

I had a hard time when I moved to North Carolina. They have a lot of roads around here where you have to turn right onto the main road then go to a U-turn if you wanted to go the the left instead. When I first moved here I thought that was the most jackassed thing I had ever seen. Then I saw how people drove and I realized why they did it. But I applied the right-on-red thinking to the left turn. I’m turning onto a one-way street at that point so why not? I got pulled over shortly after I got here for that and got a warning that while it would seem to make sense, it was still illegal.

You are leaving out a number of factors:

Is the traffic metered? When traffic is metered, the wait time for most drivers is minimized as traffic is allowed to flow.

Are left turns allowed? Even when there are left turn lanes, left turns disrupt the flow for other drivers.

Right on red decreases overall wait times unless there is significant pedestrian traffic, then it exacerbates delays.

@seawulf575 – not magnetic. Detection of current through coils in the pavement. Measure of inductance.

read.

Insufficient data; depends on details of intersection, traffic, drivers, timings, etc.

Also “spent waiting for the light to change” is not well-defined. It’s not a computer circuit, so many things involve significant delays.

For just one example, the more often the lights change, the more often people need to spend time slowing down to a stop, and speeding up from a stop.

So, in a symmetrical situation, the time spent moving in any direction is always less than 50%, because of the time needed to slow, stop, wait, and speed back up.

The only way to really tell, for each intersection and traffic situation, is to record what actually happens there. Also because even the amount of traffic trying to use an intersection is generally not predictable, and depends on everything happening all around it, even a long way away. One closed freeway ten miles away could have major effects on an intersection. And again when that freeway re-opens and releases a glut of cars.

But there will be some time where cars are traveling, and some time when cars are stopped, and some time when cars are not going full speed because they were stopped before (and the time to get up to speed again typically evolves each car waiting for the one ahead of it to notice and get moving again).

As for cars turning right, they typically slow down a little to do so, but they also remove themselves from the traffic you’re measuring. Net effect is good, even without considering that a 4-way intersection that didn’t let cars turn right would be preposterously dysfunctional.

Turning lanes help reduce the impact of turning cars on traffic flow.

I was just thinking in theoretical terms of a worst case scenario. Imagine a highway full of cars for hundreds of miles going 60 mph separated by 6 car lengths. Now imagine the same thing for an itersecting highway. It just seems to me that the traffic light at the intersection would have to allow half the time for the traffic flow of each highway. Adding to the problem would be that the cars move closer together when they stop for the red light. There might be additional delay for the cars to separate and get back to speed when the light turns green.

@elbanditoroso I’ll buy that. Thanks for the correct!

@LostInParadise Ok.

Yes, all being equal in both directions, the available time could be split evenly. And there is definitely an impact of stopping traffic in one direction, as cars will need to come to a stop, and then take time to react and accelerate when given a green light later. That takes time and energy.

The less often you stop traffic, the less often you have to suffer the effects of slowing and stopping.

But conversely, the less often you stop traffic, the more cars in the stopped direction not only have to wait, but they pile up and add more delay per car as they wait for each other in turn to get moving, and the slowest-accelerating cars limit the speeds of all of the cars behind them.

So the idea time would balance those two factors against each other.

Which is why usually lights are timed to change more frequently when there is more traffic, and less frequently when there’s less traffic.

Enough time that I think I can put sunscreen on my arms, but not enough time that I can put sunscreen on my arms.

I took a course in traffic engineering in university. There were numbers and data for everything, and I mean EVERYTHING. (I loved it.) Things like making a road 1 ft wider increased traffic flow by X%.
If traffic does not have time to flow at a light you can expect 2 cars to go through on the red. Regional differences between yellow light duration and stopping. One of the traffic engineers’ jobs was to make gaps in flow so cross traffic could be maximized – average speed, distances, to previous stops. How wide do you make the road corners to accommodate trucks, knowing the penalty of going wide is cost and lost real estate values? Will you allow a 48 ft or 53 ft trailer to turn unimpeded but require a 66 ft to touch and go over the curb corner?
I learned a lot. The value of statistics and data was evident. Anecdotal experiences were useless in the big picture.

It does sound very interesting. I would be particularly interested in the equations related to traffic flow.

You want equations? Look up Flow density and speed concentration curves. Once you get those, then you adjust them for secondary factors like lane widths, topography, road slope and layout.
Tons of fun!
I liked the calculations for “Design Speed”. You can feel if the road is correctly designed. I recall our highway system had a design speed of 85 mph.

As a slight tangent but still in topic, I heartily suggest reading this book Amazon Link

Title=“Traffic: Why We Drive the Way We do”.

It’s a bit dated (published 2008) but still excellent. I bought two copies – one was ‘borrowed’ by a friend and he still has it 10 years later.