Highway express lanes provide a faster trip for carpoolers, people who drive low-emission cars, and solo travelers who are willing to pay more.ThatÂ’s the idea, anyway.But as Americans drive more miles than ever before, express lanes are facing a challenge: they are too popular. So many drivers of all kinds are using the lanes that it is increasingly difficult for transportation officials to keep them speedy.For solo drivers, the tolls in express lanes typically vary based on demand. In some places, drivers pay more during peak travel times, and in others prices go up as more cars enter the lane....