Yesterday we pointed out that the Hammersmith public, when consulted on the proposed burying of Hammersmith Flyover, opted for the longest possible distance underground in the borough, from Hogarth Roundabout in the west to Earl’s Court in the east, thus neatly proving that the public really doesn’t like urban motorways.  The first option we examined, direct replacement on the same line with a cut and cover tunnel, clearly doesn’t meet this aspiration for the public, but what of the other option?  This is the one we’ve discussed before, the long bored tunnel from Chiswick to Earl’s Court.  The position of the western tunnel portal is more settled in the Geotechnical Report:

The horizontal alignment of the tunnels, from the western portal 450m west of Sutton Court Road, follows the A4 towards Hogarth Roundabout

That’s a bit further west than I’d originally thought, with the start of the ramps around Harvard Hill, next to a prep school and in a thoroughly well to do middle class area.  There’s a listed building, Little Sutton Cottage, 16th century, just by the portal ramps and right where any road widening would have to be carried out to fit the ramp down and roads round the outside to carry any residual surface traffic.  On the other side of the road we have the former Dairy Crest site currently being developed with a three storey Porsche after sales centre butted right up against the main road.  I can see the cranes from here.  The gap between the cottage and the new centre is about 40m, which has an access road, six lanes of A4 and a shared cycle/footpath leading to a subway in it, so fitting about eight lanes of road in there is not going to work.

There’s more room at the eastern end where the road widens around Warwick Road/Earls Court Road, to enable drivers to turn right and get stuck in Earl’s Court Road.

The options discussion has this to say on the long tunnnels:

More expensive than central Hammersmith options however, and traffic use will be limited to non-local traffic that will still have to use the existing roads. Air qualities at portals are an issue, and construction disruption at portals. The open space in Barnes, currently playing fields, would be temporarily lost during construction. The major construction disruption for this option will be focused around the portal locations which will have sections of Open Cut (220m) and Cut and Cover (225m) at each portal, in addition to this the TBM will need to exit the tunnel and be turned during construction at the portal locations which will cause greater disruption to traffic movements.

[3a] may have even less traffic than 2a, as it won’t take traffic from the North End Road area.

This is the acknowledgment of what we’d suggested previously – the longer the tunnel the higher the cost and the more surface road you have to provide for traffic that isn’t travelling from one end of the tunnel to the other.  The feasibility study also realised this:

This is a fundamental finding as traffic that joins the A4 between the start and end points of a tunnel between Chiswick and Earl’s Court will have to use a surface network. Should the flyover be removed, it would be diverted around the Hammersmith Gyratory.

Any capacity increases that can be achieved at the Hammersmith Gyratory, even if possible, would not be consistent with the vision for the improved town centre.

Here lies the flaw at the heart of the council’s loudly trumpeted public consultation – they’re pretending the public want the flyover gone when in reality they want the traffic gone.  The council, and TfL, won’t look traffic restraint in the face and therefore have to work within TfL’s barmy 14% rise in traffic to 2031 and try and find an option that satisfies both requirements.  They haven’t found one.

The short option, leaving the A4 unaltered outside the immediate town centre, is clearly unattractive to the public plus the central area still handles all the traffic that currently comes off the A4, plus whatever rise TfL put in.  The engineers are clear that cut-and-covering the A4 alone is not enough to deliver the town centre plans:

The on-line replacement of the flyover is a much cheaper option, albeit with a lot of disruption during construction. Together with the proposed remodelling of the gyratory system, this would provide the main benefits in central Hammersmith, with an open plaza between the Apollo Theatre and St Pauls church, and the opportunity for redevelopment in the central area. The downside would be that the Talgarth Road east and west [sic] of central Hammersmith would remain as it is at present.

On the long option the geotechnical report is fairly clear it just won’t work:

Surveys of the public have determined that the most popular end points for a tunnel would be between Earls Court and Sutton Park [sic] Road, i.e. option 3. However it is by no means certain whether options 2a or 3a (two lanes without junctions) would cater for sufficient traffic to enable Talgarth Road to be reduced to a lightly trafficked single carriageway. An unintended result may also be a greater volume of traffic on the gyratory. On the other hand options 2b and 3b (3 lane tunnel with junctions), while likely to divert the majority of traffic away from Talgarth Road, is considerably more expensive, not just because of the larger diameter, but because of the junctions, which involve significant lengths of disruptive open-cut and cut and cover, in populated areas.

In Hammersmith town centre the proposed master-planning, assumed to be implemented for all options, involves the closure of part of the gyratory, and a return to bi-directional traffic. While the traffic details are not known, it is considered that this remodelling will reduce the traffic capacity of the gyratory system significantly

In brief, then, the whole scheme falls down unless you reduce traffic entering the gyratory sufficiently to be able to return it to two-way working and take some of the land it currently uses to create a quality urban environment.  This cannot be achieved within TfL’s projected traffic rise as there doesn’t appear to be an engineering solution on the table that reconciles even existing levels of traffic with this aspiration.  The council is therefore lying to its constituents by pretending that all that’s needed is a tunnel and removal of the flyover – they need to remove most of the traffic and half the gyratory before that becomes a feasible option, and if you can manage that you can easily manage without the ‘flyunder’ completely.

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