There are so many good reasons to hate speed tables.
They slow down emergency response times as much as 3-16 seconds each
They can hurt patients in the backs of ambulances
They’re hard on your suspension
Because of this, they only work if you use a bunch of them in a row
We can get around some of the first two by using speed cushions. These humps have breaks in them that are are too wide for regular cars but don’t interfere with the wider wheelbases that fire trucks and ambulances use. Even so, they’re still going to have to slow down to thread the vehicle over them without hitting them.
![](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fbb63cd31-6aa8-485c-95eb-12cd4017df48_4001x2323.jpeg)
Given all these ills, you would think people wouldn’t want them. Not true. I have yet to get a traffic calming study that wasn’t envisioned as a justification for these monsters.
So why do we use them?
The short answer is that they work. If you need traffic to slow down because of the people and cyclists in the area (that the drivers can’t see), they will do the trick. It’s a bad solution because they will only work right at that location, but they do work. Because you’re using the brute force of physical laws, no one—and I mean no one—gets around their power. You can get airborne by using them, but most people don’t want to go spend $200 on a vehicle alignment afterwards.
There’s a valuable guideline in fiction writing: Don’t tell your reader; show them.
Drivers don’t see the dangers to pedestrians or cyclists because we haven’t shown them a realistic threat in terms of their day to day interactions. A rarely used trail generates no real conflicts and is no real danger to the driver (socially or otherwise) so it will not change their behavior. It’s out of their line of sight anyway.
![](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Ff4e6762f-a1d7-496e-b721-32bf7e7b72f3_700x337.webp)
An empty bike lane is just additional width—drivers aren’t stupid: they can see it’s empty. Only the desperate and foolhardy think these treatments are acceptable as a transportation solution, and accepting them only perpetuates the problem. If you want to change behavior, it takes changing the way a cross section looks and acts, which requires things like moving curb lines and adding trees that will take years to mature.
![](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6a35e803-0043-4234-a4fa-4fdd5e7333d2_1924x702.png)
Of course, the real fix is in the land use itself. It is pure fantasy to line a road with subdivision walls and expect drivers to treat it as an interactive space. The problem is that this takes time, if it ever happens at all. If you think road construction is slow, try changing suburban land use. What’s already in the ground will require all of the people who own that property to ask for a change in their HOA docs and land use regulations. Then, land use changes require lawyers, public hearings, and relaxed zoning—that can take a generation or more. Once the land use regulations are loosened, (re)building something takes even more time and economic investment from private money—and that can go either very quickly or very, very slowly.
Getting the money and political will to fix things may take a while.
When there’s real risks in specific locations, a speed table can bridge the gap until we can do what it takes to get speed down the right way.
The vision problem:
As a transportation psychology expert, let me add one additional issue for your consideration:
Where are you looking when you approach a speed table?
We’ve all learned that the faster you go, the less you see. In the perceptual narrowing study that proved it, there is a terrifying piece of information that no one seems to remember. When you’re braking, your fixations shrink to a tiny spot on the back bumper of the car in front of you. This is what it looks like in the original study—the normal viewshed of 20 degrees from center drops to 5 degrees or less:
That means that when you’re slowing down to get over that evil hump, you’re staring right at it, and nowhere else. I had a friend tell me as she was training her son to drive, he almost hit someone who was crossing about 50 feet farther down the road because he had been so intently looking at the hump that he didn’t see anything else.
You got their speed down. Good job. (My snark is showing…apologies.) Are they paying any attention to the people walking around them? Not at all—probably less than before.
Sneaky traffic engineer strategy:
Of course, that Achilles heel can also become an asset.
If I know you’re going to stare at that hump when you get there, I can use that to my advantage by putting the crosswalk on top of the speed table.
Then drivers have to look directly at the people crossing because they’re staring at the table. This is the concept of steering their attention. In other words, don’t waste your speed tables on just slowing people down. Use them to focus the driver’s attention on something they wouldn’t see otherwise.
![](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fbc613a17-930f-457c-87ef-5ddd86b8f5ef_700x523.jpeg)
I will frequently specify speed tables:
at the edges of parks
around the edges of schools
At mid-block crosswalks (always!!)
Between attractive land uses that face each other on opposite sides of the road
Get the picture? Use them to focus your driver’s eyes in the places you need them to look.
Coming up next:
I do have a series planned that gives general design guidelines based on the mental frameworks, but the weeks have been too broken up to start that yet. Maybe next week. We’ll see. I still have a pile of random design thoughts and a video or two from NOLA that may come first.
Share this with your friends. It’s been a while since I’ve sent out the mental frameworks report to my subscribers, so I’m giving fair warning ahead of time:
At the end of next week, I’m going to send out a new email to all my subscribers with the FDOT report that describes the mental frameworks and some news on where I’m speaking next and what I’m up to these days.
The report is written for design engineers, so it may not read like a page-turner, but it will work until I can talk someone into to publishing a more reader friendly version.