Rubber sidewalk slabs have been installed on top of tree roots on Rhode Island Ave. NW. Photo by Sommer Mathis. There’s a stretch of sidewalk on Rhode Island Ave., between 9th and 7th streets NW, that has long been difficult to navigate. The root system belonging to a series of large trees planted there has, over the years, led to major cracks in the concrete that continued to be pushed upward, causing something like…