Green infrastructure is often cheaper than more conventional water management strategies. Philadelphia found that its new green infrastructure plan will cost $1.2 billion over 25 years, compared with the $6 billion a gray infrastructure would have cost. The expenses for implementing green infrastructure are often smaller, planting a rain garden to deal with drainage costs less than digging tunnels and installing pipes. But even when it is not cheaper, green infrastructure still has a good long-term effect. A green roof lasts twice as long as a regular roof, and low maintenance costs of permeable pavement can make for a good long-term investment. The Iowa town of West Union determined it could save $2.5 million over the lifespan of a single parking lot by using permeable pavement instead of traditional asphalt. Green infrastructure also improves the quality of water drawn from rivers and lakes for drinking, which reduces the costs associated with purification and treatment, in some cases by more than 25 percent. And green roofs can reduce heating and cooling costs, leading to energy savings of as much as 15 percent.