Sunday, 21 January 2018

Inshes Overbridge – Width Reallocation

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The Inshes Overbridge carries Culloden Road (B9006) over the A9. It is not far from Raigmore Hospital, Beechwood, Inshes Retail Park, and Inverness Campus. It has a fairly even gradient so that many people that use it may not realise that they are crossing a bridge.

It is seen as a pinch point between two major road junctions: Inshes Roundabout and the junction to the east with slip-roads to and from the A9.

In the longer term (by 2026), Transport Scotland intends to widen the Inshes Overbridge to accommodate a 4-lane carriageway and a new desirable minimum NMU facility.

This document relates to the Highland Council’s current project, the Inshes Overbridge - Carriageway Widening scheme *, which is an immediate temporary measure intended to alleviate traffic congestion into Inverness town centre.

* It might equally well be named
The Inshes Overbridge - Cycleway Narrowing scheme.

Currently there is a shared 2-way cycle track on the north side of the road. To the east of the overbridge and crossing it, there is no footway of any kind on the south side of the road.


Width will be be reallocated from the 2-way shared cycleway to enable the carriageway to be increased from 2-way to 3-lane (2 heading into town, 1 heading out).

The resulting narrowing of the cycleway will render it sub-standard for its purpose.

This will be mitigated by two main measures:

  • the erection of two ‘Share and Care’ signs.
  • the identification of an alternative route: Inverness Campus, Golden Bridge, Raigmore, King Duncan’s Road, Millburn Road, Eastgate.

Overbridge Details

Detailed proposals have not been published but the width of the overbridge (13.3m between the parapets) is obviously a fixed constraint. The proposed three-lane carriageway will occupy a width of 9.1m (3.1m+2.9m+3.1m) from kerb to kerb.The length of the overbridge is about 25m.

This arrangement will be achieved by reallocating 0.5m of width from the cycleway to the carriageway.

Impact on Cycleway

The RSA Report refers to Transport Scotland’s document Cycling by Design. (The section on Width Requirements for Cycleways is 6.1.4 starting from page 62.)

It recommends a Desirable Width of 3.0m and an Absolute Minimum of 2.0m. The current full width is about 3m but varies along the route.

Section 6.1.5 lists additional clearances. The two that apply for the overbridge:

  • 0.5m for a continuous feature >1.2m or a bridge parapet of any height
  • 0.5m for being adjacent to a live carriageway

Thus the Effective Width of the current cycleway is reduced to the Absolute Minimum of 2m. It (just) satisfies the guidance.

The proposal to reallocate 0.5m from the cycleway to the carriageway will reduce the Effective Width to 1.5m.

There is a note in the guidance saying that, with low flows, a width of 1.5m may be considered over short distances where no alternative is available.

This is open to interpretation. What is a short distance?
It seems reasonable that a short distance occurs around a local obstruction such as a lighting column or a data cabinet. Cyclists and pedestrians can see each other and negotiate past these.

Is the bridge length of about 25m to be considered short? (It might take a cyclist 5 seconds or a pedestrian 15 seconds to travel that distance.)

In addition, there are two separate lighting columns intruding 0.4m into the edge of the cycleway. These are local obstructions and briefly reduce the Effective Width below the Absolute Minimum of 2m (but above 1.5m) thus conforming to the guidance. Reallocating 0.5m from the cycleway will reduce the Effective Width here to about 1.25m which is well below even the exceptional Absolute Minimum of 1.5m.

The claim that “no alternative is available” is difficult to justify when the alternative already exists and will be removed by the new road layout.

Reference Documents

17 Dec 2017  [14 pages]
Stage 1 & 2 Road Safety Audit Report (RSA)
Produced by Wylie:Lodge Road Safety Consultants

for The Highland Council

13 Dec 2017  [21 pages]
Cycle Audit
Produced by The Highland Council

 [HCC - 21 Jan 2018 - Comments welcome]