Calculating Intersection Sight Distance in Neighborhoods

IntersectionThe City of Bloomington, Minnesota has the most proactive traffic engineering department I know of.  They review the traffic related police reports once per week and keep their own dataset of reported traffic incidents (many of which don’t have the severity to make it into the official state crash database).  They also have a proactive street auditing system to look for potential problems at intersections before there’s a crash problem.

Because they have such fine-tuned “crash” data, they have a lower threshold for investigating intersections that may have potential safety issues.  An intersection that has one reported crash every two years wouldn’t rise up for a review in most cities, but an intersection with a crash every two years for eight to ten years would raise a flag in Bloomington’s system.

They don’t place stop signs at neighborhood intersections unless they’re absolutely needed and they also believe in using yield signs instead of always jumping to a stop sign.  One thing they look at closely when reviewing an intersection is the sight distance.  They look at the sight distance two ways –

  1. They draw a triangle for each corner going 30 feet back from the intersection along each approach (the cross hatched area on the aerial).
  2. They draw the approach sight distance based on the speed limit (the empty space triangle).

Based on their experience, they keep the 30 foot triangle clear.  This often means having residents remove shrubs or trim trees.  You’ll see on the empty space triangles that there are often things like houses or garages in the approach triangles.  Houses routinely sit on the edge of these triangles and do not significantly impact safety.  But it is a factor to scrutinize if the sight line blockage is more than typical and there is a trend of some crashes at the intersection.  Over time, city staff are learning how much blockage in the approach sight distance is too much and influences higher than normal crash rates.

I really like this two pronged approach to sight distance at neighborhood intersections.  I strongly believe in keeping the 30 foot triangle clear and keeping neighborhood intersections uncontrolled as much as possible.  But, if a house is right on the corner in a hilly neighborhood I’d recommend putting yield signs on one of the roads to clearly identify the right-of-way.  If the sight distance is blocked on both roads, stop signs may be needed.

And if you’re working on a new subdivision, try to work the grading and building placement to avoid needing stop signs.  Sight distance isn’t always a factor for the site designers and city staff should check it as part of the review process.

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