A permanent gravel requires a gradually sloping crown surface to promote good drainage and the ability to channel water away from the eroding gravel. In some places, the driveway is secured with large stones to keep the gravel in the driveway. Since most gravel driveways are less than one centimetre, they are best suited as a cement substrate. Crushed gravel with its high surface area and high water content serves as a good drainage gravel for driveways. Crushed gravel can also serve as the best drainage of gravel in a driveway, as it can cause less erosion and less damage to the roadway. Gravel driveways are a fraction of the cost, long-lasting, able to withstand very heavy vehicles, and do not weather away easily due to exterior elements.