Lane space is often limited, particularly on major corridors that carry large commuter volumes but have little room to expand. Take the Sydney Harbour Bridge as an example. It moves huge volumes of traffic each day within a very constrained corridor, and tidal flow lanes (sometimes called contraflow or reversible lanes) are used to reallocate that limited space depending on peak demand.
But there’s a safety trade off…
Research by Manuel et al. (2020) found an 18% increase in injury crash risk with reversible lanes (not specifically the Sydney Harbour Bridge, but a meta-analysis). Results varied significantly depending on the site design.
Common issues include:
- Incorrect turns
- Entering the wrong lane
- Misreading lane control signals
- Turning into or from the wrong lane
- Late lane changes
A notable crash type involves vehicles turning across opposing traffic from the wrong lane.
So what helps reduce the risk?
- Predictable peak direction (clear tidal patterns)
- Clear and consistent signage
- Restricted and simplified turning movements
- Fewer time dependent rules
- Avoiding use through intersections (especially complex ones)
- Careful design of entry and exit points
But physical separation isn’t off the cards. Impressive systems like a movable barrier (zipper system) can remove much of the risk. Check out this video on Auckland’s moving barriers.
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