Design of Subways for Pedestrians' Crossing

Subways (or underpasses) and footbridges are alternate options to at grade pedestrian crossings. In the hot climatic conditions, the footbridges might not be suitable for local environments, but subways could be a useful option.

Subways provide full segregation of pedestrians and vehicular movement and are potentially the safest form of crossing facility. They can provide high level of safety (with the provision of appropriate measures like CCTV, lighting etc.) with minimum disruption to traffic flow. However, the usage can be poor if they are not properly designed.

Factors to consider

Many factors affect the choice of whether to provide a subway or not. Factors useful in considering the provision of a subway include:

• Volume of pedestrian traffic
• Volume of cycle traffic
• Type of land use to which access is to be provided
• Speed of vehicles along the road and the volume of the traffic including the proportion of heavy goods vehicles
• Location, convenience and safety of alternative routes for pedestrians and cyclists
• Use by elderly, visually impaired, disabled people including wheel-chair users and people with prams and pushchairs
• Environmental aspects
• Other aspects particularly relevant to the local situation
• Cost of subway
• Effect of change on local land use over the next 15 years including any prospective recreational routes for pedestrians and cyclists.

Siting of subway

The line of the subway should be close to the main line of travel for the majority of subway users in order to maximise its use. The subway should be as short as possible. Where the number of pedestrians is large, an option might be to raise the level of the road and reduce the height and length of pedestrian stairs and ramps.

Types of subway

Subways may be designed for use by pedestrians only or by pedestrians and cyclists. Subways for joint use should normally be segregated and preferably by level difference. However, an un-segregated shared surface for both pedestrians and cyclists may be suitable in some situations.

Personal security aspects are extremely important for encouraging use of the subway. Wide approaches, subway alignments with good through visibility and good lighting, all within the view of passing pedestrians and passing traffic, will help to minimise pedestrians’ fears for their personal safety. Subways and their accesses should be designed to avoid places of concealment in the interests of personal security.

Motorcycle barriers may be necessary in some locations to prevent cars and motorcycle being driven into subways or subway approaches.

Cross-section of subways for pedestrians only

Three types of pedestrian subway may be used:

• A wide section, suitable for situations where a subway forms an extension to a footpath system not less than 5.0m wide carrying large number of pedestrians or where for aesthetic reasons the normal section is not considered suitable.
• A normal section, suitable for the majority of situations.
• A narrow section, for situations with small number of pedestrians where the normal section could not be justified on cost grounds.

Sight distances of 4.0m or more should be provided at corners and changes of direction. For calculation purposes, pedestrians can be assumed to be 0.4m away from an adjacent vertical wall. The visibility envelope should extend from an adjacent vertical wall. The visibility envelope should extend from a height of 1.5m representative of an adult to 0.6m for a child. Inside corners rounded off to a radius of 4.6m will meet this criterion.

Cross-sections of subway for combined use

Pedestrians and cyclists can share the same subway and associated ramps. For combined use to be successful, the existing travel lines and those expected in the future should be investigated for both pedestrians and cyclists. Short diversions of one mode may be necessary to encourage the other mode to use the dual facility.

Segregated Subways: The width for pedestrians’ path should be segregated from the width for cyclists’ path, preferable by level differences as shown below.

Alternatively, segregation can be achieved by means of guard railing which would serve as a physical barrier to separate the footpath users from cycle rack users. Where these measures are not suitable, a raised dividing line and tactile paving should be provided to assist visually impaired people. The minimum dimensions for cross sections are given in following table.

Stopping sight distance for cyclists given in the following table should be provided within the subway and on the approaches.

The distances are applicable to design speeds of 10 km/h or less on sharp curves and straights. The geometric requirements for cyclists’ facilities will in these cases also affect the dimensions of the pedestrians’ facilities. The design speeds are not significantly affected by gradient. For layout purposes, the line of sight of a cyclist should be taken from a point 1.5m high, and at least 0.6m away from the edge of the cycle track. The design of subway walls, wing walls, associated ancillary earthworks and landscape works should take account of these visibility requirements.

Un-segregated Subways: When the total number of pedestrians and cyclists is small, an un-segregated subway may be acceptable, particularly for short subways with good though visibility. The minimum dimensions for cross sections are given in the following table.

At sites where space is restricted or where the total number of pedestrians and cyclists is very small, the subway width may be reduced to 3.0m.

An alternative where few cyclists are expected is to provide a narrow pedestrian subway. Signs would be required to indicate that the cyclists should dismount before entering the subway and that no cycling is permitted with the subway. It would also be necessary to ensure that the cycle track is legally terminated either side of the subway.

Access

Access to subways may be by way of ramps or stairs which may be straight or helical. Consideration should be given to providing both ramps and stairs to suit able-bodied, cyclists people with prams and push chairs, those with heavy shopping or luggage visually impaired people and disabled people including wheelchair users.

Access ramps or stairs should normally be the same width as the subway, except when multiple ramps and stairs are connected to a single subway, they may be narrower.

The thresholds of all subway accesses, tops and bottoms of flights of stairs, should be provided with a system of tactile paving to assist visually impaired people.

Access ramps: Ramps should be allowed to run in to the subway beyond the threshold as there could be a risk of cyclists hitting the soffit of the subway.

Landings should be provided at changes of direction and changes of gradient. Landings should be used, even on straight ramps, so that the total rise between landings is less than 3.5m. Landings should normally be of the same as the ramp and 2.0m or more long measured along the centre line of the landing. All landing should be approximately horizontal and appropriately drained.

Pedestrians’ ramps: Gradients of 5% or less are preferred for access ramps where significant numbers of disabled persons or heavily laden shoppers are expected to use the subway. In other situations, gradients shallower than 8% are preferred but gradients up to 10% are permitted for short lengths in exceptionally difficult sites. Stepped ramps may also be considered at exceptionally difficult sites, although wheelchair users find stepped ramps difficult to negotiate.

Cycle ramps: from the point of view of limiting speed and reducing cycling effort, access ramp gradients should preferably not exceed 3%, with a maximum of 5% or 7% in restricted spaces. In the steeper gradients barriers to encourage slower speeds until they clear the steep ramp should be considered. Staggered barriers may help to reduce speeds.