This post is dedicated to a ubiquitous but unsung hero of modern transportation infrastructure: the raised pavement marker, or “dot.”
These markers — known in California as “Botts’ dots” after their inventor — are those ubiquitous round or square lumps used as lane markers on roadways all across the Fruited Plain. At first glance, they’re pretty simple. But this apparently simplicity belies a lot of engineering prowess.
Construction
First of all, what are they made of? Well, they come in multiple shapes, and are made of different materials. For an up-close-and-personal look at these little guys, I spoke to Ken Dinning of Professional Pavement Products in Houston, who was gracious enough to tell me everything there is to know about raised pavement markers.
City streets tend to use simple round buttons, measuring 4″ across, like this:
It’s basically just a dome-shaped lump of fired ceramic clay, painted with nonreflective paint and then glazed. It’s 4″ in diameter and 3/4″ thick. The bottom isn’t painted, and has ridges molded into it, giving it a larger surface area for adhesion to the road, which is accomplished using a bituminous glue. (More on that later.) That’s the basic, no-frills marker.
Now we come to the real star of the show, the Class R Raised Retroreflective Pavement Marker. Avery Dennison is the largest manufacturer of Class B dots, but this model is a Glowlite 987, made by the friendly Communists at the Chongqing Universal Pavement Marker Company:
As you can see, the construction of a Class B dot is a lot different. It’s a hard ABS plastic shell, filled with a material kinda like concrete. It measures 4″ square and weighs 8 oz. It has a trapezoidal cross-section with a reflector on at least one of the two sloped faces. The reflector, angled at 30° for maximum visibility, exhibits a property called retroreflectivity, meaning all the light shone into the reflector reflects back directly to the source of the light, not in some other direction. No matter what angle you look at the thing, you’ll see the same bright reflection.
Because this type of dot has multiple parts, the colors can be customized in all sorts of ways. The shells come in yellow, white, blue, red and green. The reflectors come in the same colors, and one dot can have two different-colored reflectors. Each color combination has a different application.
For a centerline on a two-way street, the typical dot is a yellow shell with two yellow reflectors. For a lane or shoulder marker, it’s a white shell with a white reflector. But for one-way streets, the lane markers use multiple colors. If you’re driving down a one-way street, you’ll see white reflectors. But the back side — the side you’ll see in the rear-view mirror — has a red reflector, serving as a “wrong way” warning to dumbasses.
Every so often, you’ll see a stray blue dot stuck in the middle of a lane all by its lonesome, with a blue reflector in each direction. This little guy is a silent sentinel of public safety, marking the location of a fire hydrant.
Cost
According to Ken, a typical Class B dot costs about $2 when bought in bulk. But as any Home Depot shopper will tell you, the material price is meaningless. They get you on the labor and adhesives, and the same thing is true with the dot business. The “all-in” price of a dot — including the dot itself, adhesive and installation — is about five bucks.
Installation
Now that you know what the dots are and how much they cost, it’s time to stick ‘em to the road. Highway department use either epoxy or bituminous adhesive, which is similar to roof tar. Texas uses bituminous adhesive, and the specifications for this stuff are pretty demanding. The same adhesive is used for concrete and asphalt roads, and can be applied when the temperature of the road is anywhere from 40°F to 160°F. It has to withstand 200°F temperatures without softening. Dots are not afraid of global warming.
This means you have to heat the stuff to very high temperatures in order to apply it. The adhesive comes in 50-pound and 60-pound blocks, which are fed into a machine that heats it up to around 400°F. The machine crawls along the road and squirts gobs of adhesive at the right intervals. Workers then apply the dots by hand or machine, wiping the reflector lenses with paint-thinner to remove any wayward adhesive. Dots aren’t applied over expansion joints. Florida’s specifications mandate that no more than 2 percent of the dots should come loose or misaligned in the first 45 days of traffic exposure.
Testing
Dots are subjected to a battery of tests that boggles the mind. In California, they’re tested for identification and workmanship, bond strength, glaze thickness, hardness, directional reflectance, index of yellowness, color, autoclave, strength by compressive force and water absorption.
These tests are quite thorough and quite destructive. The dots are examined, manhandled, pulled with machines, shattered with hammers, dipped in hydroflouric acid, baked in ovens, immersed in water, scuffed with steel wool and crushed with 5,000 pounds of direct force.
Only the toughest and strongest dot recruits will be permitted to stand their eternal watch in the highways and byways of the Golden State.
Use and Abuse
In addition to providing visual clues for motorists, their raised nature provides tactile and aural feedback. We’ve all drifted over the line, only to be jerked back to attentive driving by the whump-whump-whump of a sequence of dots.
However, having bumps on the road presents a problem in cold climates — snowplows routinely scrape dots right off the road. In California, standard dots are countersunk in small depressions in the road. However, this is expensive to do, and it reduces the visibility of the dots. Accordingly, manufacturers have developed “snowplowable” dots. 3M’s plowable dot looks like this:
These dots are set in a cast-iron fitting, flush with the roadway or a bit lower. The flat edges along the sides guide a snowplow blade safety over the reflector housing, allowing a close shave every time.
Well, that’s it. Everything you could ever want to know about dots. Thanks to Ken Dinning of Professional Pavement Products for showing me around his store.
BONUS KNOWLEDGE: I learned another interesting fact about the traffic-control business. Speed bumps — “traffic calmers” in the business — are available that will slow down a car, but not an ambulance or fire truck. They’re built just narrower than the width between the tires on an emergency vehicle. Pretty cool, huh?
