Sight Triangles

Just about everyone who drives knows that intersections with limited visibility are dangerous. But a lot of people haven’t worked through why they’re so dangerous.

It’s not just the obstruction. It’s the way the human mind responds to it.

What we’re talking about here is an obstruction in the “sight triangle.”

(Until a concept is given a name, it tends to hang out in the periphery of accessible knowledge. Not consistently retrievable. Difficult to explain to others.)

Here's an obstructed sight triangle:

If there’s something in that triangle that obstructs the drivers’s view, there’s a real problem.

The obstruction can be really anything including:

• Parked Vehicle

• Trees

• Hedges

• Fences

• Walls

• Buildings

• Infrastructure

A big part of that problem when there’s an obstruction in the sight triangle comes from the way our brains are wired.

When we pull toward an intersection we’re checking for cars. That’s our task.

We aren’t checking for obstructions. That’s not our task.

When we don’t see a car coming, we assume it’s safe (rather than thinking that there’s something blocking our view).

When something is blocking our view the obstruction creates a “false negative.” We feel confident it’s okay to proceed. We don’t feel a sense of uncertainty.

This is explained by a couple of factors:

• The visual system fills in missing information automatically.

• An obstruction created by a hedge does not trigger a “warning signal” the way glare or darkness does.

• There’s no internal alarm for an absence of information.

To the driver, the scene feels complete, not incomplete. Looking is (falsely) equated with seeing and perceiving.

I remember one of my first sight triangle cases. Both drivers were nice, honest, etc. Both said they looked but didn’t see the other vehicle. I went out to the scene and realized there was a big hedge right at one of the corners of the uncontrolled intersection. That explained everything. And the homeowner’s insurance carrier paid its policy limits.

A lot of traffic safety improvements are expensive. I was reading an article in The Washington Post and it said that speed bumps cost $5,000 and roundabouts costs about $170,000.

But fixing a sight triangle issue—especially if it involves bushes or trees—is essentially free. We have a case right now with the City of Puyallup. Here’s how it works there:

• You let the City know that a property owner has a sight triangle obstruction.

• The City contacts the owner. The owner has two weeks to fix the problem.

• If the owner doesn’t, the City will go out, limb the tree or trim the hedge, and then charge the owner.

Simple and inexpensive/free (where the cost is imposed on the property owner who should pay it anyway).

I bet on the route you drive every day there is at least one sight triangle obstruction. Does safety matter enough to you to report it?

If it does, send me the details. I will send a letter to the property owner asking them to correct the problem and to send me a photo of it when it's fixed. If they don't do that I'll write to the municipality that has jurisdiction over the property, put it on notice, and ask that it take action to remove the obstruction.