Why did the toad cross the road? To get to the other side, of course. But also, to reproduce.
Nearly 4,000 toads, frogs and newts have been rescued as they tried to cross one of only five roads closed for the migration season in the UK each year to reach a breeding lake on the other side.
The patrol of Charlcombe Lane, near Bath, has recorded more than 50,000 toads, frogs and newts in the last 22 years on a half-mile stretch.
Here is an older article about a toad tunnel in what sounds like a similar situation. The Charlecombe Lane site is described as being half a mile long, so I’d expect that the same issues would apply.
As they mention in the older article, as well as the tunnel itself, this involved ditches to funnel the toads towards the tunnel - so, even if it would work, it would be a sizable operation to install it, and so would be expensive - though whether the cost alone would be the issue, I really don’t know.