Grade separation

Description

Grade separation means infrastructure that puts pedestrians and other road users including rail at different heights. This typically includes pedestrian underpasses (tunnels and subways) and overpasses (footbridges and elevated walkways). This overcomes issues regarding greater travel distances for pedestrians using such facilities.

Grade separation can include under- and overpasses for motor vehicles with the pedestrian route remaining at-grade. Under and overpasses are often designed to accommodate cyclists as well as pedestrians.

An underpass and grade separated walkway are shown in the photos below.

Underpass under Harewood Road / Russley Road roundabout, Christchurch. (Photo: Jeanette Ward)

Use of a disused motorway ramp as a shared path, providing grade separated pathway. (Photo: Jeanette Ward)

When to use

Check whether grade separation is a suitable treatment for your site by referring to the following guidance:

PNG: Crossing selection process

Benefits

Allows pedestrians to cross unhindered by vehicle traffic.
Can reduce walking travel time by eliminating waiting time.
Can reduce vehicle delays (if replacing an at-grade facility).
Can be covered for weather protection.

Implications

May increase the risk if pedestrians continue to cross at-grade.
Can increase pedestrians’ travel time due to requirement to change level or other detours.
Can result in personal security concerns because of reduced natural surveillance from traffic.
Costly to construct.
Can be visually intrusive.
Gradients and/or steps can create difficulties for mobility impaired pedestrians.

Recommended parameters

Primary Safe System treatment as pedestrians are separated from vehicle traffic[1].
Grade separated route must be more convenient to pedestrians than any other option (use topography to minimise grade changes for users).
If cost prohibitive, consider reducing vehicle speeds so other crossing types become feasible.

A comparison of the advantages and disadvantages of overpasses and underpasses are outlined in the table below.

Table: Overpass and underpass considerations for pedestrians

Overpasses	Underpasses
generally require greater vertical separation than underpasses due to the clearances for overheight vehicles, and therefore, longer approach ramps and greater travel distances. can provide attractive views and visual connections with adjacent land uses resulting in a safer and more pleasant experience for pedestrians. can be made visually appealing. are more likely to be open to the weather and the risk of objects being thrown onto the roadway. are usually cheaper to construct than underpasses in an existing environment.	are generally perceived as providing less personal security than overpasses due to lower natural surveillance. can have drainage or watertable issues. naturally provide weather protection. generally require less level change for pedestrians. sightlines on approaches can be difficult to achieve and are particularly important when the underpass is shared with higher speed users (eg cyclists).

Overpasses

Underpasses

generally require greater vertical separation than underpasses due to the clearances for overheight vehicles, and therefore, longer approach ramps and greater travel distances.
can provide attractive views and visual connections with adjacent land uses resulting in a safer and more pleasant experience for pedestrians.
can be made visually appealing.
are more likely to be open to the weather and the risk of objects being thrown onto the roadway.
are usually cheaper to construct than underpasses in an existing environment.

are generally perceived as providing less personal security than overpasses due to lower natural surveillance.
can have drainage or watertable issues.
naturally provide weather protection.
generally require less level change for pedestrians.
sightlines on approaches can be difficult to achieve and are particularly important when the underpass is shared with higher speed users (eg cyclists).

Legal considerations

A pedestrian or rider of a mobility device must use an underpass or a footbridge when one is reasonably available to the pedestrian for that purpose within a distance of 20m (Road User Rule, 11.3(1)).

Land Transport (Road User) Rule 2004

Design considerations

Grade separated crossings are most effective when they are more convenient for pedestrians than at-grade crossings. Pedestrians should ideally stay at the same grade when crossing or have only a minor change in level. The road could be elevated or sunk to reduce the amount of elevation change for pedestrians. In planning for new areas where a grade-separated crossing is required, it may be possible to utilise the terrain to achieve this. If this is not possible, ramps and steps that comply with standards are required.

PNG: Ramps and stairs

Both over- and underpasses can result in longer walking journeys than at-grade crossings – and they are unlikely to be used where the walking distance is more than 50 percent greater than the at-grade distance. Even when less than this, some pedestrians will try to take the shortest route which may be across an unsafe location. Improving the convenience and aesthetics of the grade-separated option and potentially installing fencing may be necessary. Fencing should be continuous, unclimbable and long enough to prevent people walking around the ends.

Pedestrians can be concerned for their personal security at underpasses and overpasses, particularly if they are not well used.

PNG: Safety from other people in Pedestrian Network Characteristics

To overcome this:

structures should be well lit, potentially on a continuous basis
pedestrians should always be able to see their whole route without any obstructions or recesses, and (where possible) from a public place some distance away
skylights should be provided in underpasses
the route should include direction signs
each entry/exit should have passive surveillance from adjacent buildings or public space
CCTV installations may be helpful.

Overpasses or underpasses may require the relocation of utilities. They can also provide a conduit for utilities.

Further guidance on the design of overpasses and underpasses for pedestrians can be found in the following:

Bridging the gap: NZTA Urban Design Guidelines

Design elements

Many dimensions for overpasses and underpasses are determined by specific site conditions. The table below provides some general dimensions.

Table: Overpass and underpass design elements

Feature	Value	Further information
Width	At least 2.4m	It should be greater where the route is shared with other users (eg cyclists, scooters, etc).
Overhead clearance	Desired minimum 2.4m, absolute minimum 2.1m	Greater clearance can help make the overpass/underpass feel more open.
Grade change	No more than 6.5m for overpasses No more than 3.5m for underpasses	There may be exceptions where constraints mean these grade changes need to be greater, eg. an overpass over electrified railway lines.
Roadway clearance	At least 4.9m (6.5m on overdimension routes) for overpasses only.	See Overdimension route information in PNG: Requirements for other road users
Approach ramps	PNG: Ramps and stairs
Access control devices	Provide access control devices if necessary.	Access control devices on paths design guidance note
Lighting	Ensure lighting does not impact the overhead clearance.	Bridging the gap: NZTA urban design guidelines PNG: Lighting

Appropriate emergency vehicle and maintenance vehicle access should also be provided.

Reference

[1] Corben, B. (2020). Integrating Safe System with Movement and Place for Vulnerable Road Users(external link), Austroads, AP-R611-20, p19

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