The design process for road restraint systems includes carrying out a risk assessment to identify the hazards and minimise the risks to the road users.
The risk assessment is documented by using an Excel spreadsheet, a copy of which can be obtained from the Government's website https://www.gov.uk/guidance/standards-for-highways-online-resources
A user-guide is also available on the same web-page.
TD 19/06 also directs the designer to use BS 6779 and BS 7818 for the design of specific elements of parapets.
BS 6779: 1998 - Highway Parapets for Bridges and Other Structures.
Part 1: Metal Parapets for the provision of infill to parapets (see TD 19/06 clause 4.29, 4.39, 4.40)
Part 2: Concrete Parapets for the design of reinforced concrete parapets with some amendments (see TD 19/06 clauses 4.56 to 4.60)
Part 4: Reinforced and Unreinforced Masonry Parapets to assess the containment capacity of existing masonry parapets (see TD 19/06 clause 4.62)
BS 7818: 1995: Pedestrian Metal Parapets
TD 19/06 clause 9.3 says that the manufacture and installation of pedestrian restraint systems must conform with BS 7818 until such times as the drafting of prEN 1317-6 is completed. However EN 1317-6 has been superseded by PD CEN/TR 16949:2016 and, by inference, should be adopted instead of BS 7818.
Parapets are designed and tested by manufactures who apply to the Highways Agency to be included on an Approved List. A copy of the the "List of EN 1317 compliant road restraint systems" can be obtained from the Government website https://www.gov.uk/government/uploads/system/uploads