They look like ordinary concrete blocks, but highways engineers reckon they will save a full year of disruption at Junction 15 of the M4.

The 29 pre-cast concrete blocks have been installed under the bridge over Day House Lane, which carries the A419 north of the junction.

They will be used as bridge supports and eventually an underpass – but they mean the bridge will not have to be knocked down and work can continue with the road staying open.

They will also allow pedestrian and cyclist access under the bridge – something that demolishing, infilling and rebuilding the road may not have done.

The work is part of a multi-million-pound scheme to cut congestion at the junction, with the dedicated London-bound slip road joining the M4 set to open in October.

The junction improvements are being carried out by Highways England contractors – with the £17.5m cost including a £4.8m contribution from Swindon Borough Council, £3m from the Swindon & Wiltshire Local Enterprise Partnership and £4.3m from Redrow Homes and Persimmon Homes, as well as £5m from Highways England’s growth and housing fund.

Swindon Borough Council’s cabinet member for strategic infrastructure, transport and planning Gary Sumner said: “The upgrades to Junction 15 of the M4 are absolutely crucial as they will strengthen our excellent transport links benefitting not just businesses already based here, but also new employers who are looking to relocate to a fantastic location along the M4 corridor.

“The new culvert under the A419 has also transformed Day House Lane into a ‘green route’ which will allow pedestrians, cyclists and horse riders to use the new route from Badbury and Liddington to get to Coate Water.”

“We are investing more than £100m in Swindon’s road network to futureproof it for years to come. The new and improved junction will have extra capacity and it has been great to see the new dedicated lane for London-bound traffic take shape as that will make a tremendous difference to motorists.

Mark Fox, the south west head of scheme delivery for Highways England, said: “The installation of pre-cast concrete sections has meant not only a saving of millions of pounds, but also drastically reduced disruption to motorists in the area, which is something that we always strive for.

“As the scheme progresses, we will make every effort to ensure that the impact on the local community is kept to an absolute minimum, and we will communicate any changes via roadside signage.”