An artist's impression of the Cambridgeshire guided busway
A ceremony is being held to mark the official start of work on one of the world's biggest guided busways.
The 25km guideway along the disused St Ives to Cambridge railway line will link Huntingdon and Cambridge.
The government is providing £92m towards the cost of the £116m scheme which aims to cut congestion on the A14 and in the city centre.
However, some opponents to the scheme have called instead for a train service to be reintroduced along the line.
The work starting on Monday involves investigating the old railway viaduct over the River Ouse near St Ives.
A drilling machine will be put on to the viaduct to assess the strength of the bridge foundations.
Bob Menzies, head of delivery for the guided busway, said: "We need to find out more about how the viaduct was built and its current condition.
"Then we can go about strengthening it and bring it up to current standards."
The whole of the guided busway is expected to be open in spring 2009.