Part of the experiment is to test the traction surface. They had a problem with a bicyclist slipping, so they switched from stick-on surface to spray-on surface

Part of the experiment is to test the traction surface. They had a problem with a bicyclist slipping, so they switched from stick-on surface to spray-on surface. The article says they use an angled surface and natural precipitation to keep the glass clean. I can only imagine how much dirt builds up on a highway over a day. A similar complication would be dirt or sand being blown onto the roadway by wind, or washed there by rainfall.