For reasons of safety and ease of maintenance, Washington and dozens of other communities are building rubber sidewalks made 1 ground-up tires of cars and bikes. The rubber squares are up to three times more expensive than concrete squares but 2 longer, because tree roots and freezing weather won"t crack them. That, 3 , could reduce the number of slip-and-fall complaints made 4 uneven pavements. The shock-absorbing surface also happens to be easier on the joints of slow runners and more forgiving when someone slips or falls. And the rubber sidewalks are considered more environmentally friendly. They 5 a way to recycle some of the estimated 290 million tires 6 out each year in the United States, and they do not restrict tree roots the way concrete squares 7 . Since 2001, a company, Rubber Sidewalks, has been grinding thousands of old tires into small pieces, 8 sticky substances and baking the material into sidewalk sections that weigh less than eleven pounds a square foot, or a quarter of the weight of concrete. The rubber squares are now 9 in two colors of gray and orange. The District of Columbia has spent about $60,000 to replace broken concrete with the rubber squares here and there in a residential 7 northeast of the capital.