In 2012, national parks received nearly 283 million recreational visitors. When the number of non-recreational visits (vendors, commuters, etc.) are added, it is easy to see that the park transportation system is under an ever-increasing burden.
As the number of visits increases, park roads will increasingly experience traffic congestion. The NPS has not been building new roads or adding lanes to alleviate congestion. Instead, it has been pursuing alternative transportation systems, which frequently include high occupancy vehicles (shuttle buses, trolleys, ferries) as a means to add capacity, alleviate congestion, improve the visitor experience, and protect our resources.