It is important to point out that the position of the tunnel portal is still officially under consultation but the only tunnel plan offered has its western portal in line with the winter solstice sunset as seen from Stonehenge. Mike Pitts says of this plan that it: "scored highly on its OUV impact"... "OUV. That’s “outstanding universal value”, a concept that is taken very seriously in assessing any changes in the landscape. How would a tunnel portal so close to the stones affect OUV, bearing in mind floodlights at night (if such things were visible, they’d be a problem all year round, not just on midwinter day)? I’d say very badly."
The OUV component that concerns the sightlines is discussed on the UNESCO page http://www2.astronomicalheritage.net/index.php/show-entity?identity=49&idsubentity=1
The integrity of sightlines within the Stonehenge WHPIn assessing the integrity of these sightlines today, we make the assumption that they were largely kept clear in the Neolithic and Bronze Ages, so that the monuments could be used in the way in which we presume they were used, with the sun or moon rising or setting behind distant horizons visible from the monuments themselves. The sightlines are shown in (this figure).
Sightline from Stonehenge looking southwest (midwinter sunset)There is a growing consensus that the midwinter sightline was more important than the midsummer one, as discussed above. Today the integrity of this sightline, and its intermediate ridge lines and final horizon, is marred. Looking out from Stonehenge, the first problem is the A303 (0.5 km), which runs relatively close to the monument, and presents a considerable visual and noise intrusion to this alignment. Moving further south-west, the round barrow known as the Sun Barrow—which is on the alignment and on the Normanton Down ridge line—is intact (0.9 km), but the sightline then quickly runs into the plantation known as Normanton Gorse (1.1 km), which obscures it. Still further south-west is another plantation known as The Diamond (2.2 km), before the alignment continues towards the place that would form the visible horizon from Stonehenge in the absence of intervening vegetation, at Oatlands Hill to the west of the A360 road (and outside the WHP) (4.4 km). This horizon is also obscured by yet another plantation, at The Park. The sightline probably ends at the site of a much later Iron-Age/Romano-British settlement. It is difficult to determine the exact place because the various obstructions mean that we must rely upon computer modelling.
As closely as I can I have overlaid the Ordnance Survey map with the position of the portal (marked within a red circle) and the UNESCO Southwest Sightline (thick red line). My calculation is that the centre of the sightline is 60m from the centre of the portal. With the margin of error, the width of the sightline and the size of a tunnel portal I would say that is pretty close to the portal being on the sightline, too close for my liking.
Click pictures to enlarge.
It is worth pointing out that it isn't just the central line at Stonehenge. The Station Stones also align to the midwinter sunset so one could fairly consider the solstitial line at the monument to be nearly 100m wide.