Government guidance is that National Parks should be afforded the highest protection.  This is outlined in English National Parks and the Broads: UK Government Vision and Circular 2010

Paragraph 31 on major development states:

Major development in or adjacent to the boundary of a Park can have a significant impact on the qualities for which they were designated. Government planning policy towards the Parks is that major development should not take place within a Park except in exceptional circumstances… Applications for all major developments should be subject to the most rigorous examination and proposals should be demonstrated to be in the public interest before being allowed to proceed… The Government expects all public authorities with responsibility for the regulation of development in the Parks to apply the test rigorously, liaising together to ensure that it is well understood by developers.

Paragraphs 85 – 87 place a particular emphasis on prioritising sustainable transport first and foremost above any consideration of road building in a National Park and that routes should be developed which divert traffic away from National Parks, not into and through them:

85. Improvements of main routes through the Parks are governed largely by considerations outside those relating to the Park area itself. However, there is a strong presumption against any significant road widening or the building of new roads through a Park, unless it can be shown there are compelling reasons for the new or enhanced capacity and with any benefits outweighing the costs very significantly. Any investment in trunk roads should be directed to developing routes for long distance traffic which avoid the Parks.

86. In exceptional cases where new road capacity were deemed necessary, a thorough assessment would be needed on the loss in environmental value resulting from any new infrastructure. This would need to be accompanied by measures to minimise any damage and where possible measures to enhance other aspects of the environment. This would include measures to compensate for the loss of environmental or landscape value to local communities and users of the Park, as well as measures to enhance local access to services or sustainable access to points of interest that may be detrimentally affected by the new infrastructure. The Department for Transport would expect that, in addition to the statutory environmental bodies, the Authorities are consulted by the highway authority, or in the case of trunk roads by the Highways Agency, at an early stage in the design of any road and traffic management schemes within or potentially affecting Parks.

87. When assessing options for dealing with increasing demand for access to and within Parks, Transport Authorities are expected to have considered demand management measures before new infrastructure is considered. Where new transport capacity is deemed necessary, consideration should be given to the scope for sustainable low carbon transport initiatives prior to consideration of schemes to create more capacity for car access. These could include schemes to enhance public transport, provide car club and sharing schemes, or improve segregated cycling and walking connections between train stations, local towns, villages and car parks and the local rights of way network. The Government has published guidance on sustainable travel which gives more information and best practice examples [29]. Where increased access to points of interest or beauty spots is deemed necessary, measures may need to be taken to regulate entry and exit at peak times to minimise the detrimental effect on the local environment caused by any additional traffic. Government expects complimentary measures for non-car users to be put in place in parallel to any car based measures. For any transport schemes, all practicable measures for mitigating impacts on the natural beauty and special qualities of the surrounding area and on local communities should be put in place.

[29] Delivering sustainable, low carbon, travel: An essential guide for Local Authorities. 9 November 2009