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Cycling by Design

4. GEOMETRIC DESIGN

Introduction

4.1 To develop a consistent and acceptable standard of cycling provision it is necessary to identify design standards which may be used to complement the network hierarchy introduced in Chapter 3. The following Chapters present a series of guidelines, ranging from what is currently regarded as ‘best practice’ in terms of desirable minimum standards through to absolute minimum standards which can be used in instances where desirable standards may not be appropriate, achievable or affordable.

4.2 There may be instances where, for economic or environmental reasons, not even the application of an absolute minimum standard is considered appropriate. In such instances Design Organisations should record the reasons for not adopting the absolute minimum standard together with any compensatory measure (e.g. warning sign, high skid resistance surfacing etc) considered essential for the safety of users of the facility.

Design Speed

4.3 A range of cycling design speeds are proposed. These relate to the network hierarchy introduced in Chapter 3 as follows:-

(i) Long Distance Routes : 30kph

(ii) Commuter Routes : 25kph

(iii) Local Access Routes : 20kph

Sight Distance

4.4 The available distance over which the cyclist has visibility to potential hazards, approaching traffic or junctions is a critical design feature.

4.5 When designing for the cyclist three visibility parameters should be assessed:-

(i) Sight Distance in Motion;

(ii) Stopping Sight Distance (SSD); and

(iii) Approach Sight Distance.

4.6 Sight Distance in Motion: Sight Distance in Motion is the distance a cyclist requires to see along a cycle facility to make the task of riding feel safe and comfortable. The distances specified in Table 4.1 are based on European research which indicates that the Sight Distance in Motion should be the distance covered by the cyclist in 8 seconds.

4.7 Stopping Sight Distance: SSD is the distance required to perceive, react and stop safely in adverse conditions (i.e. The distance covered in the perception/reaction time plus the actual braking distance). The SSD values introduced in Table 4.1 are based on a perception/reaction time of 2 seconds and a comfortable deceleration rate of approximately 0.15g.

Table 4.1: Geometric Design Parameters

4.8 Junction Visibility: Where a cycling facility joins a motorised carriageway the minimum cycle facility junction visibility requirements are set out in DMRB Volume 6, TD 42/95 : Geometric Design Major/Minor Priority Junctions.

4.9 The cyclist should be provided with the same ‘y’ distance value along the adjoining road as a motorised vehicle in order to maximise safety. Refer to Figure 4.1, Tables 4.2 and 4.3 below.

Figure 4.1: Visibility Splays for Priority Junctions within the Road Network

Figure 4.2: Visibility Splays for Cycle Facility Junctions within a Cycle Network

Table 4.2: Visibility Splays for Priority Junctions and Crossings

Table 4.3: Cyclist Visibility Splays - X values

4.10 Where two cycling facilities join the minimum junction visibility requirements shall be in accordance with Figure 4.2 and Table 4.3.

Horizontal Alignment

4.11 Shared Road and Cycle Lane Facilities: Other than in exceptional circumstances the horizontal radii found within the existing road network should be in excess of any minimum cyclist requirements therefore no specific guidance is provided.

4.12 Where exceptional circumstances occur, the cycle track standards identified below should be applied.

4.13 Cycle Tracks: To be able to cycle comfortably horizontal radii are required which may be negotiated without loss of speed. Refer to Table 4.1 for individual design speed requirements.

4.14 In the vicinity of junctions smaller radii may be used as cycle speeds reduce. An Absolute Minimum Radius of 4.0m is recommended. If lower radii are used a cyclist’s speed is liable to fall below 12kph making it difficult for some cyclists to maintain balance.

Vertical Alignment

4.15 Cycle facilities are likely to be in areas with relatively low gradients therefore vertical curvature is unlikely to be an issue. As a guide the following values are provided.

4.16 Crest Values: Vertical Crest Curve Values are based on the Sight Distance in Motion values detailed in Paragraph 4.6 combined with an eye height of 1.5m and object height of 0.26m. These values are recommended as Desirable Minimum Crest Curves in Table 4.1.

4.17 A further set of Crest Curves, recommended as Absolute Minimum Values, have been developed from a combination of the SSD values presented in Paragraph 4.7 and general comfort criteria.

4.18 Sag Values: Sag values for motorised vehicles are generally based on comfort criteria. However due to the comparatively low speeds envisaged, less than 30kph, it is not likely to be a controlling factor when designing for the cyclist.

4.19 Therefore, provided that the cycle facility has a generally smooth profile, it is not considered necessary to specify Sag Values for cycle design as the minima would be difficult to construct.

Gradient and Crossfall/Superelevation

4.20 Gradients: In general terms the lower the longitudinal gradient the more attractive a cycle route will be to the general user.

4.21 Table 4.4 introduces a range of gradients which are currently considered to be appropriate. The final choice of gradient is likely to be controlled by local conditions.

4.22 Crossfall and Superelevation: On new construction the designer should aim to provide crossfall of 2.5%. If necessary superelevation may be applied to a maximum value of 7% in areas of low horizontal radii.

Table 4.4: Gradients

COMMENTS : CHAPTER 4

Cycling by Design’s prime objective is to draw together and rationalise existing cycling design guidelines into a single comprehensive, coherent reference document that may be used as a source of sound technical advice. In order to achieve this objective it is imperative that Cycling by Design is developed through consultation. Please provide any comments or suggested amendments you may wish to put forward with respect to the preceding chapter.

Comments/Proposed Amendments:

Please send your comments to:

The Scottish Executive Development Department,

Standards Traffic & Environment,

Area 2-E,

Victoria Quay,

Edinburgh EH6 6QQ.

On or before 31^st December 2000.

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