Highways England is costing out plans to widen the proposed A303 Stonehenge Tunnel to create a hard shoulder on the dual carriageway scheme.
Consultants are being asked to investigate the impact of adding emergency refuges or an additional hard shoulder in order to increase tunnel safety over the 3km bored tunnels.
The decision comes as the Government is due to make a decision on the provision of more emergency refuge areas on UK Smart motorways amid concerns about safety.
Work is being undertaken six months after three consortia were shortlisted to bid for the £1.7bn project to ease congestion on the A303 between Amesbury and Berwick Down and allow traffic to travel under the World Heritage Site.
If Highways England decides to go-ahead with the 11th-hour safety changes it would impact existing budgets for the project.
The present baseline scheme is for an all-purpose dual carriageway tunnel without refuges or hard shoulder.
Consultants are being asked to cost the cheaper option of two laybys per tunnel, spaced at 1000m centres, constructed using sprayed concrete lining techniques, or increasing the diameter of the tunnels to incorporate a hard shoulder lane over the entire length of the tunnel.