As a primary route to southwestern England, the A303 is frequently congested on its single carriageway sections. It passes through the Stonehenge World Heritage Site and the Blackdown Hills Area of Outstanding Natural Beauty, and attempts to upgrade the road in those areas have been controversial. In particular, the Stonehenge tunnel, which would see the A303 rerouted underground, has been proposed and delayed several times. Nevertheless, the road remains a popular alternative to motorway driving.