Urban planning community | #theplannerlife

1. ## Vision triangles

I'm trying to understand the underlying rationale for requiring Vision Triangles at controlled intersections. City code looks for 110 feet of sight line even though the car/bicycle needs to come to a "complete" stop. Should the sight line be based where the car's driver is located at the full stop? If it were a yield sign or such, I could see the increased length but the existing code seems excessive especially in lieu of a controlled intersection, ie. stop sign. TIA

2. Originally posted by jkbrown
I'm trying to understand the underlying rationale for requiring Vision Triangles at controlled intersections.
To have unobstructed view of the roadway.

Originally posted by jkbrown
Should the sight line be based where the car's driver is located at the full stop?
The jurisdiction usually decides on the sight triangle location, depending upon several factors from AASHTO/DOT guidance. Normally it is from a point presumed to be from full stop.

3. Even though it is a controlled intersection, the requirement for a vision triangle takes into account the way people drive. How many times do you see someone think that a yellow light means they need to speed up, even so that they are entering the intersection when it is already red? How many times do you see somebody think that a right on red does not require them to come to a stop? Most drivers are adiots. Improved sightlines are there to mitigate the effect of their stupidity.

4. Thanks for the responses. I found what I needed was the departure sight triangle from a controlled intersection. Its slightly different and makes use of the "time gap" to determine the nominal distance for departure sight.

Cardinal, interesting thought on the yellow light drivers. I'm waiting for the longer period of time between the actual red and the other directions being given the green light. Kinda of clearing the space.

5. Originally posted by Cardinal
Improved sightlines are there to mitigate the effect of their stupidity.
But one has to ask: do sightlines enable their stupidity? Sightlines make you feel safe, and feeling safe makes you take bigger risks. I know I'm certainly a lot more cautious at the downtown intersections where historic building placement has made the sightlines bad. But then I'm being anecdotal. I'm sure someone has done the numbers (I hope!) to show that these huge vision triangles reduce collisions.

6. WRT sight triangles, I'll just say that in addition to irresponsible or bad drivers, traffic planning also has to consider the whole range of folks in/on vehicles out there (including the elderly, very short people, people driving Smart Cars, cyclists, pedestrians, you name it). Having narrowly escaped certain death by an old lady in an historic area who did not see me coming because she was so short she couldn't see over the fire hydrant on the corner (which is also next to an old building right up at the street), I am sensitive to the questionable driving habits of the old and short. Granted it was not a controlled interesection, but still - that blue hair almost killed me!