You might think painting a crosswalk is only slightly more complicated than putting a yellow line down the middle of the road. Take a look around at the faded and nearly invisible crosswalks in your community and you can begin to appreciate that it’s not that simple. I had the opportunity to watch a demonstration of a new surfacing technology a few weeks ago in Boston and found it fascinating to literally watch paint dry…no, seriously!
Nature and automobile are hard on Boston streets. I’d argue a greater hazard to cyclists than cars is the unpredictable but uniformly bad condition of pavement where potholes appear even before construction of new roads and bridges ends. The roadway is in a constant state of flux. To attempt to apply a layer of an additional material (paint) on that surface is destined for eventual failure.
The solution I saw demonstrated is called StreetPrintXD. At 6 1/2 minutes, my homemade video is long–if you want to see a short, complete install demo video, you can watch the one produced by Integrated Paving Solutions instead. But mine does put it in some perspective as you can see the workers installing on a busy Boston street in 90-degree heat.
First, the asphalt surface must be evaluated–if the road is more than a year or two old, you will need to do a “mill and fill”–to lay down a fresh coat of asphalt where the crosswalk will be. In the video, you can see how the roadway around the crosswalk is older and lighter.
The mill-and-fill is a big deal that will add to the cost of any project because a machine must be rented and traffic disrupted for half a day…and of course you will have to hire a police detail.
Once the surface is ready, a heater is used to soften the asphalt so the paint will stick. The “paint” is aggregate-reinforced thermoplastic. By incorporating aggregate into a plastic mixture, the material is made more like the asphalt surface in terms of its wear characteristics. Regular thermoplastics have glass beads instead of aggregate which mean they will move differently under stress and lack the irregular reinforcing characteristics of roadway materials.
Once the asphalt is soft, rather than simply apply the coating, a mesh template is hammered into the pavement. In the final product, this creates a look of brick, but it also creates valleys of indentation that will be shielded from the immediate friction of automobile traffic. This feature is a key attribute of another product, DuraTherm, which has been used to create more elaborate, patterned crosswalks instead of the traditional brick look. These patterns hold up well because the pattern is impressed into the roadway so car tires do not contact it.
After stamping out the pattern, the coating is applied as big plastic sheets, heated, and stamped down into the indentions in the pavement.
A coating of sand is applied while the coating is still wet to more closely match the traction of the asphalt. Maintaining traction is critical for safety–remember the Tour de France time trials in the rain a few years ago with cyclists spinning out of control on pavement markings? The key is to maintain the same coefficient of friction across the surface–not to create either an extra slippery or suddenly extra sticky surface. Changes in the coefficient of friction turn the momentum of a moving object into unexpected horizontal forces that can start a skid.
The final product is expected to last as long as the roadway surface.
Most crosswalks will not be able to afford this treatment. In the video above, we are looking at Melnea Cass Blvd which sees 30,000 vehicles per day. When it’s not that busy, it’s probably being salted, sanded, and plowed. So a quick spray of Krylon isn’t going to last a day out there.
But even on our less busy town streets, I see how quickly the traditional paint jobs fade. Each spring our DPW goes back to repaint lines and crosswalks as needed. Roughly speaking, for a 2-lane road, the treatment above should cost about $5-10,000 including the preparation work. A newly-paved road would be cheaper–the actual materials cost is $13-17/square foot and if you avoid the mill-and-fill, it makes everything faster and easier.
A fair question to ask–especially given this cost–is “why bother?” In the case of our town, we don’t have the funds and are applying for a grant. But the objective is to get the attention of motorists who blow through crosswalks even when school crossing guards are present. We have “knockdown” signs in the road which do in fact get “knocked down” all the time. Perhaps if we only deployed the beacons I blogged about yesterday, it might make enough of a difference. But part of the goal is to increase awareness and respect for walkers overall. As the paint fades we face a long winter/spring season before repainting eventually happens…unless it rains too much that year. If we could make a permanent infrastructure enhancement in the most critical locations, I believe we could begin to affect behavior norms by telling everyone: this crosswalk is here to stay.