And that is exactly the reason why I can't imagine a Haverhill-Cambridge railway service being feasible. A busway combined with a P&R near the A11/A1307 junction would work rather well, I feel.....although the ability to bypass traffic by using the existing Southern busway would be hampered by Hills Rd bridge preventing 'deckers being used. 
I agree, Adam.
The one advantage which a restored Cambridge-Haverhill railway would have, would be avoiding road congestion by joining the Great Eastern line south of Gt Shelford and accessing the new Addenbrooke's railway station if/when it's built, especially if it were part of a through Cambridge-Braintree route. However, the option of a through route via the Colne Valley looks a non-starter in terms of the amount of the former trackbed lost to the south of Haverhill. Nor is it clear that there would be sufficient demand for such through traffic.
The main commuter flow is Haverhill-Cambridge and a busway using some of the former trackbed looks eminently do-able, given that buses can cope with steeper gradients and tighter curves.
One would suspect that it would start at the A1307/A1017 roundabout NW of Haverhill. It could follow much of the old trackbed via Bartlow, or run parallel to the A1307 to Linton and pick up the old trackbed there. Buses wouldn't encounter the problems faced by a railway in crossing the A11 & A505. A bus service could serve the south of Granta Park and use the existing road-link to/from the A11/A505. A further short busway section, on an entirely new alignment could serve the Babraham Institute. Both would give further public transport benefits to commuters. Neither of these would be achievable with a restored railway.
Similarly, I think that the only way the northern busway could have worked as a re-opened railway would've been as a through route project to the ECML at Huntingdon. But I think CAST.IRON were rather naïve in proposing this.
I couldn't imagine the traffic demand given that (especially with the work being done to correct the historic mistake at Ely north junction) there's plenty of capacity on the existing Cambridge-ECML link via March. Moreover, so much of the former line from St Ives to Huntingdon has been lost that it would have been a massive civil engineering project – to build, in effect, a new railway – for dubious benefit, financial or otherwise.
And that would have done little too help give an attractive commuting alternative to the private car from St Ives and Huntingdon/villages around into central Cambridge, and also put in public transport infrastructure ahead of the Northstowe development.