how do traffic apps get their info ?

who called it ?

post #2

/thread

 

Careful man... I'm prepping a special drone just for you.

It's got the new and improved anal probe option

 

oh balls

 

it ain't yer balls ya gotta worry about Pete.....

 

Ain't this great? The OP is getting more information about traffic control than he could have ever dreamed, we're awesome here at TW

 

Okay... Since I'm sure at some point, Colt is going to ask "Why do I always get stuck at red lights? Are they trying to make me buy more fuel?"

I'll just repost this now and maybe avoid that entire thread....

One "driver" of signal timing is pedestrian speed. 3.5 feet per second.
Walk time is to be enough to get from the button to the curb, but we use a minimum 7 seconds.
Flashing Don't Walk is to be enough to cross the street (at 3.5 fps, most people walk at around 4.5), but the vehicular yellow and all-red clearance can be considered, so we can trim as much as 6 seconds at some locations. So for example, on a 100ft crossing with a 4 second yellow and 1 second all-red, our minimum time allocated in the coordination cycle would be 31 seconds green time.

It takes 4 seconds for the average driver to react to a green light and get into the intersection, so minimum green is 4 seconds for a minor, 7 seconds for an arterial.
Once the first car is moving, we assume 2 seconds per car.
We look at our count data.... if we need to move 1600 cars per hour, and we have two lanes, then we need on average (1600/2)*2 seconds of green time per hour.... or about 27 minutes for that direction.
Sounds like plenty, and it might be if the cross street is a little residential street that only cycles once every 15 minutes.
But if we have two major streets and they both carry that volume.... now we're screwed. That's 54 minutes of green just to move the cars, and we still need time for the left turns, for the yellow and red clearance intervals, and the initial queue startup, so we have 2 seconds, plus 3.5-4.5 yellow, plus 1-2 all red... that's 8 seconds "lost" every time the light changes. It's time to see the engineer and discuss the possibility of adding a lane.

So we try to balance our cycle times. A shorter cycle time keeps everyone moving and reduces the number of cars waiting when the light goes green, so the public perceives that there is "less traffic", but it costs us 8 seconds per direction every cycle. On a 60 second cycle time, that's about 15 minutes every hour... 25% of our time with nobody moving (other than those entering on yellow). A longer cycle time reduces time lost to clearance intervals, but at the same time, it builds longer queues and people complain when they wait what, in their opinion, is "too long" and when they see 5-10 cars in front of them and are nervous they won't "make" the green.
The secondary problem is, the guy on the little residential street DOES wait "too long" because we have to synchronize the entire street based on what the largest intersections are doing... so if we need a 2-minute cycle at a large intersection, we're stuck with a 2-minute cycle at the small ones.
IF there are no crosswalks, or if the streets are narrow enough so that we can fit a full cycle with pedestrians into a 1-minute cycle, then we can "double cycle" the small intersections. Some people on the arterial will have to stop, but at least half will make it through. We've got one coordination plan downtown where one large intersection has us screwed up, so we run it on 105 seconds, and the surrounding on 70, so they are in sync once every 3 cycles for the minors and every-other cycle for the big one (105*2=210 / 70*3=210)



Now try doing that with 6 intersections that are only 500 feet apart. You can't coordinate both directions at the same time. You have to determine the direction of heaviest demand at each particular time of day and adjust the offsets to allow for the most free flow in that direction. Sometimes we can get an "okay" flow the other way, other times they are completely hosed and stopping at every light.

Now do the same thing cross-coordinating on a grid.

 

I think my brain just imploded.

 

This thread is so full of win!

 

LOL!

That's the kind of shit I do every day.

Well... that... and fixing the fuckups of the last engineer.

We have three sets of laws...

Laws of physics.
Laws of the land.
Laws of politics.

One we can't violate, one we shouldn't violate, and the other we have to violate without getting fired.

 

Inrix