Introduction and purpose of geometric design
The basic objectives in geometric design are to optimize efficiency and safety while minimizing cost and environmental damage. Geometric design also affects an emerging fifth objective called liveability, which is defined as designing roads to foster broader community goals, including providing access to employment, schools, businesses and residences, accommodate a range of travel modes such as walking, bicycling, transit, and automobiles, and minimizing fuel use, emissions and environmental damage. [The Role of FHWA Programs in Liveability: State of Practice Summary – U.S. Department of Transportation]
Residential road geometry
Table 3 provides information on the general geometry and usage for internal residential roads. Residential access roads are generally conventional cross-section roads with separate provision for motor vehicles and active travel users. On a Shared Surface Residential Access all users navigate a common surface.
Major Residential Access Road (Primary Streets) | Residential Access Road (Secondary & Local Streets) | Shared Surface Residential Access (Tertiary Street) | |
---|---|---|---|
Type of use Also refer to Active Travel Design Layouts |
Typically provide a primarily movement function. Road between strategic routes or linking urban centres. Primarily segregated active travel routes. Bus access is likely |
Both movement and place function. Multi-modal corridor with pedestrian, cycle, bus and other motor vehicle activity. Active travel interventions will depend on traffic flows and speeds. |
Strong place function. Emphasis on pedestrian and cyclist use. Not acceptable for use as a bus route |
Single point of access dwelling limits |
1000 Normally no more than 400 from a single point of access. This threshold can only be exceeded based on a robust evidenced justification to be agreed with the LHA. |
400 Normally no more than 150 from a single point of access. This threshold can only be exceeded based on a robust evidenced justification to be agreed with the LHA. |
50 Normally no more than 25 from a single point of access. This threshold can only be exceeded based on a robust evidenced justification to be agreed with the LHA. |
Access to schools | Yes |
Yes, but not as a cul-de-sac.
|
No |
85th percentile design speed | 20mph | 20 mph | 15mph |
Shared surface | No | No | Yes |
|
Carriageway width 6.75m |
Carriageway width 4.8m up to 50 dwellings and 5.5m for 50 to 400 dwellings Except on a bus route where the carriageway should be a minimum of 6m wide (subject to tracking assessment) or on a road serving a school where the carriageway should be 6.75m wide in all cases. |
Overall highway corridor width of 7.5m required to accommodate all road users, green infrastructure and utility equipment.
|
Where a road is to be narrowed, to help control vehicle speeds, for example, the minimum carriageway width (kerb to kerb) = 3.7m. Minimum lane width at a restriction, such as pedestrian refuge in the middle of the road = 3.2m. | |||
Centre-line radius | Defined by tracking | Defined by tracking | Defined by tracking |
Crossfall | 1:40 | 1:40 | 1:40 |
Longitudinal gradient |
Flexible surfacing minimum: 1:100 Block surfacing minimum: 1:80 In all cases maximum: 1:20 In all cases, at junctions: not to exceed 1:30 for first 10m of the side road |
Flexible surfacing minimum: 1:100 Block surfacing minimum: 1:80 In all cases maximum: 1:20 In all cases, at junctions: not to exceed 1:30 for first 10m of the side road |
Flexible surfacing minimum: 1:100 Block surfacing minimum: 1:80 In all cases maximum: 1:20 In all cases, at junctions: not to exceed 1:30 for first 10m of the side road |
Vertical curves | See vertical curves section | See vertical curves section | See vertical curves section |
Visibility distance at junctions, bends and vertical crests |
25m | 25m | 17m |
Cycles and footways | Please refer to tables in the Active Travel Section of LHDG |
||
Verges |
Grassed verges minimum 1m wide, minimum area 10sqm. Hard paving otherwise. Minimum 2m width to accommodate sufficient space for tree planting. |
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Steps | Not normally acceptable in areas to be adopted as public highway unless a suitable alternative ramp is provided for those unable to climb steps. |
Pavement parking
Highway layouts that encourage pavement parking must be avoided. The council will apply its experience and knowledge from best practice and lessons learnt from existing development during the assessment of designs and will resist adoption of proposals where it considered that pavement parking is likely to become an issue.
Designing roads layouts that serve schools
Parking in the vicinity of schools, as children are dropped-off or collected, is a safety hazard and can cause traffic congestion.
For new residential developments, the need for a new school on the site and its planned location must be established at the master planning stage in consultation with the council and the Local Planning Authority (LPA), see the Highway Development Management section. This will avoid future issues related to road safety and traffic congestion and ensure that the adoptable carriageway width standard of 6.75m is adhered to on school access roads.
The design of highways adjacent to new and existing school sites must:
- Encourage active travel options;
- provide ‘safe routes to school’; and
- minimise the risk of on-street parking problems.
These measures will need to be considered as part of the transport assessment for the development, alongside the requirement for a school travel plan. The same requirements also apply to proposals for the expansion of an existing school and provision of new highway close to an existing school site.
Whilst the council will seek to ensure that sustainable transport opportunities are taken up through the planning process, it is recognised that there will still be a demand for drop-off and pick-up to school by car. Accordingly, the council will seek to ensure that safe off-street drop-off / pick-up provision is provided for at school sites.
Shared surfaces
Manual for Streets (MfS) suggests that shared surfaces work well in short lengths (doesn’t require motor vehicle dominant features) or where they form cul-de-sacs, where traffic is less than 100 vehicles per hour, and where parking is controlled. Care must be taken in the design of shared-surface layouts to ensure that the development’s whole design, including building type and layout and use of street furniture conveys to users the nature of the area. Motor vehicles should not dominate, and the layout should not simply appear to be a road without footways.
It is also important that any shared surface is designed for safe use by people with visual impairments and that they include an alternative means for visually impaired people to navigate by. In accordance with MfS, developers must consult with relevant representative groups and access officers in designing proposals. Guidance in the Royal National Institute of Blind People's (RNIB) Key Principles of Inclusive Street Design, helps designers make positive decisions about provision for people with sight related problems and other disabilities.
The type of surfacing materials will normally be a secondary feature in defining the nature of the area. It is not normally acceptable to simply use a different material to convey the nature of an area to users. The council will assess the proposed surfacing material for any shared-surface area in terms of safety and effectiveness of design, including proposed housing layouts.
Setbacks
In addition to accommodating the needs of all road users, green infrastructure and utility equipment, where buildings front directly onto the highway outward-opening windows, drainage downpipes and other adjacent facilities, should be set back at least 0.5m behind the proposed highway boundary.
Inclusive highway
Principle 6 “Supporting Inclusive Highway” sets out the council’s values regarding expectations for the delivery of accessible highway. This principle applies to all road types including those serving employment and commercial properties. Developers must ensure that they are meeting the statutory duty under the Equality Act 2010 through the design of highway layouts. It is strongly recommended that during the design process reference is made the Department for Transport’s “Inclusive Mobility” document and RNIB’s Key Principles of Inclusive Street Design.
In exceptional circumstances the council may consider a relaxation of the 1:20 longitudinal gradient standard on sites with particularly difficult topography. However, relaxations must be a last resort for longitudinal design. The impacts of the development on more vulnerable users must be fully assessed by the developer where this is being proposed. The financial cost of cut/fill is not a material consideration when assessing the ability to achieve gradients to aid active travel options.