Plans to reduce motorway congestion by opening up more hard shoulders will put pressure on surface water drainage, resulting in a new design challenge for highways engineers.

Modern roads are designed so that there is a gradient from the centre to the edge, to aid water run-off. Traditionally, gullies then work with the kerb to transport and collect rainfall run-off into a highway’s drainage system. This type of system can result in standing water collecting in the hard shoulder lane, owing to the spacing between gullies, particularly in flatter areas.

Historically this would not have been a problem, because cars are not often on the hard shoulder and when they are, they are usually either travelling at a low speed or are stationary. Now, with plans to open up more hard shoulders, this standing water suddenly becomes an issue.

Rainwater on a carriageway reduces the effectiveness of tyre grip; spray from the rainwater gets thrown up by car tyres, which reduces visibility, and drag on car tyres from rainwater ponding can alter the balance of vehicles, which can cause skidding.

When frozen, any water that has seeped through or under the surface expands, pulling apart the carriageway’s construction, which then deteriorates during freeze-thaw cycles. By opening up the hard shoulder, you are also subjecting drainage ironwork – a common failure point - to heavy traffic. As such, from both a safety and maintenance perspective, the quick, effective removal of rainwater on carriageways is paramount.

In answer to this, highway agencies and local authorities are turning to combined kerb and drainage solutions. Not only are solutions such as this far more cost-effective - when compared to a solution that requires separate kerbs, gullies and underground pipework – but they also offer significant benefits for the asset owner.

Combined kerb drainage removes water along its length thus avoiding ponding. It also removes the ironwork associated with gullies, and water is kept closer to the surface, making discharge easier.

With congestion an increasing issue and the removal of hard shoulders being introduced as one solution to this, local authorities and highways agencies will need to be mindful of the consequences.

Re-thinking the design of drainage on carriageways from both a safety and maintenance perspective should be high on the agenda.

Dr David Smoker is business development manager at ACO