Framework 3: No nearby people, no behavior change

A dose of reality for VZ speed management

Here’s the MF3 recap: Speed choice is directly tied to how wide or open the corridor looks, but just getting it narrow won’t necessarily keep speed down. If you want speed to change, you need people there within visual range, which means getting them within the driver’s PPP, the 60’ corridor that they are most often looking at. No people, no change.

If you need any proof, I’ve got a doozy for you.

This is a 38 foot wide visual tunnel and I’m not gaining on the guy in front of me. This is a pretty typical speed for this gorgeous corridor. It is a best case scenario for motion parallax effects. Motion parallax is the 3D effect you get when you’re driving by objects and the ones that are closer move by faster than the ones that are far away. Motion parallax helps you “feel” how fast you are going and this feels fast. There’s no desire on my part (or anyone else’s) to choose to go slow. There’s no realistic risk to anyone but myself and unless I’m drunk, it’s not a problem to keep my car between the curbs.

This is a dose of reality that we desperately need in our speed management and vision zero work. Its implications are vast and unavoidable.

Implication 1: Changing the sign doesn’t change driver behavior much.

The most obvious implication is that changing a posted speed limit is not going to make a big difference—our data indicates changing the speed limit sign by 10 mph is likely to have a 1.8 mph change in the 85th percentile speed of the corridor. Strict enforcement works while the drivers can see the enforcer is there. In our field testing, we had a few locations that were strictly enforced and the drivers couldn’t know where the police officers were waiting—I know: I’ve gotten a ticket there myself. The wild thing was that drivers still used the speed that the formula predicted, which was not what was posted, and everyone used nearly exactly that speed. Unlike most roads where there was about a 6-8 mph scatter, this was a 3-4 mph scatter. I’m convinced most drivers rarely know or care what is posted. They drive what they see in front of them.

Implication 2: You’ve got 4 lanes to work with at most.

Remember this figure?

What this implies is that if your visual tunnel is more than 60’ wide, you are probably not going to get freewill 85th percentile speeds below 30 mph. Narrower is no guarantee (you need people there), but if it’s wider, you’re no longer in the game. Let’s look at a 4-lane cross section:

It’s tight, but no real problem here. Once you take into account the curbs, you’re left with 11’ lanes and 6’ sidewalks (or 10’/8’). Everyone gets seen.

Once you go to 5 lanes, you’re out of luck. Once you add in curbs, you’re really down to about 3’ of sidewalk for each side. The guy in the turn lane might be able to see what’s going on in the sidewalk, maybe—but he probably won’t care. The people in the inside lanes might be able to see what’s on their sidewalk, if there’s not another car in the way, but probably won’t notice what’s going on from the other side.

One of the most interesting things we noticed was that on-street parking impacts were all over the place. If they didn’t get used regularly, drivers would go faster because it was just more open space. When they were used, drivers slowed down, but their focus was more on the road and multitasking didn’t shift. Interacting with people replaces the dopamine hit drivers get from distractions.

Implication 3: Overdriving your interactive vision

I mentioned this yesterday, but I want to give you a better picture so you can see what I’m talking about.

Drivers tend to focus within an area in front of them that ranges from 1 second to 2 seconds in advance of their position. At 30 mph, the drivers entire focal area remains within that 90 foot useful cone—the 60’ wide PPP we talked about yesterday. By 40 mph, half of their focal area is outside of the 90’ reach where drivers can decode all facial expressions. At some point between those two speeds, drivers apparently quit trying to interact. Beyond 45 mph, 2/3 of their focus is outside the PPP area. Landscape architects have intuitively recognized that between 30 and 40 mph, drivers shift their interactivity and the walkability in the area evaporates.

This is a classic feedback relationship. Pedestrians can tell that drivers aren’t looking for them anymore, so they don’t want any part of that. Drivers don’t see pedestrians there as often, so they don’t feel the need to slow down—but that is the crux of the 4th mental framework, which we will cover later this week. You can reverse this, but as you can guess, it’s going to take some hard work.