§ Lord Marlesfordasked Her Majesty's Governemnt:
What criteria they use to determine whether new bypasses on A roads should be single carriageway or dual carriageway; whether annual average daily traffic flow is one such criteria; and, if so, what threshold determines whether a bypass will be a single or dual carriageway. [HL3678]
§ Lord Davies of Oldham:The criteria used for determining the level of provision for a road are based on the five over-arching objectives for transport—environment, safety, economy, accessibility and integration, defined by the New Approach To Appraisal (NATA).
To support the appraisal, a traffic assessment is undertaken in which the annual average daily traffic flow in the scheme's opening year is used. Technical Advice Note TA46, which provides advice on determining carriageway provision, states that a single carriageway may be appropriate for an opening year traffic flow of up to 21,000 vehicles per day while the minimum opening year traffic flow for a dual carriageway would be 11,000 vehicles per day. However, the level of provision adopted for a particular scheme is based on consideration of a range of factors in NATA including forecast traffic growth, operational efficiency, safety, cost and environmental impact.
The New Approach To Appraisal(NATA) is published electronically on the Department of Transport's WebTAG website http://www.webtag.org.uk/
Technical Advice Note TA46 is published by the Highways Agency in the Design Manual for Roads and Bridges.