The lights will be in place on the B2149 between the two villages for approximately nine weeks starting on Monday, August 8.
This is to enable a new junction to be constructed as part of a new northern access route onto the Havant Thicket Reservoir site.
This route will be used by construction vehicles while the reservoir is being built, keeping the majority of this traffic away from residential areas in Havant and Rowlands Castle.
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It will also become the main access point for visitors from outside the local area to Havant Thicket Reservoir once the reservoir is completed in 2029.
Bob Taylor, Portsmouth Water CEO, said: ‘I would like to say a big thank you to everyone living and working near to the Havant Thicket Reservoir site.
‘As a relatively small and community-focused water company, the support of local people means so much to us and we are incredibly grateful for your patience and understanding as these improvements to the B2149 begin.
‘We sincerely apologise in advance for any inconvenience caused by these works.
‘We are continuing to work closely with stakeholders, including local councillors, community groups and environmental organisations and are doing all we can to minimise disruption and take local views into account.’
The CEO added that he believes the controversial reservoir will bring ‘community, social and economic benefits to the area’, and said: ‘This is in addition to the reservoir’s central purpose - to protect world-renowned chalk streams in Hampshire by providing an alternative, sustainable source of water to meet the needs of a growing population.’
The temporary traffic lights will allow traffic to move along the B2149 while one half of the carriageway is closed for work to take place.
However, a full road closure will be needed once the new junction is finished in place, to allow for resurfacing and re-profiling of the B2149 carriageway.
The closure is necessary for safety reasons, with further details of the timings and duration to be issued closer to the time.
All of these works to the B2149 have been agreed with Hampshire County Council.