From the Highways England video https://www.youtube.com/watch?v=ZapaIn_ZI6M I have grabbed a screen and overlain the narrowest possible interpretation of the solstitial alignment view that UNESCO says should be protected from any building. I would argue that the sightline should be at least from the width of the monument (the station stones are also markers of the alignment) and by any measure of sanity a lot wider than that. It shows it is not just road lighting we need to worry about it is also vehicle headlights.
Highways England consultation page about the Stonehenge Tunnel for more details and to send them feedback.
From UNESCO's Full Description (IAU Extended Case Study format): Stonehenge World Heritage Property, United Kingdom
A retrospective Statement of Outstanding Universal Value (SOUV) was prepared for the Property by the State Party in 2011 and has been approved by UNESCO. The relevant parts of that SOUV in relation to astronomy are quoted here:
Statement of Significance
The Stonehenge, Avebury and Associated Sites WHP is internationally important for its complexes of outstanding prehistoric monuments…
They provide an insight into the mortuary and ceremonial practices of the period, and are evidence of prehistoric technology, architecture and astronomy…
The complexes of monuments at Stonehenge and Avebury provide an exceptional insight into the funerary and ceremonial practices in Britain in the Neolithic and Bronze Age. Together with their settings and associated sites, they form landscapes without parallel.
The design, position and inter-relationship of the monuments and sites are evidence of a wealthy and highly organised prehistoric society able to impose its concepts on the environment. An outstanding example is the alignment of the Stonehenge Avenue (probably a processional route) and Stonehenge stone circle on the axis of the midsummer sunrise and midwinter sunset, indicating their ceremonial and astronomical character…
UNESCO’s Astronomy and World Heritage Initiative adds: "Assuming that these were once largely clear in the Neolithic and Bronze Ages, it is important to try and ensure that the sightlines are as clear as possible today. All plans should ensure that no further development takes place along them."
Stonehenge and Avebury World Heritage Site Management Plan 2015 lays out various policies including:
Policy 3c – Maintain and enhance the setting of monuments and sites in the landscape and their interrelationships and astronomical alignments with particular attention given to achieving an appropriate landscape setting for the monuments and the WHS itself