The Agency confirmed this week that Robertson's contract would not be renewed after he signed a one-year extension last year.
Speculation is already mounting as to who will be his successor with some engineers calling for an internal candidate to follow on work already started to implement the recommendations of the Nichols Review of the Agency's major roads programme.
Such an appointment would put chief highway engineer Ginny Clark and national traffic director Derek Turner in the frame to succeed Robertson.
"In the past they haven't looked at internal candidates, but it should be considered quite seriously because the agency has been through a lot in terms of developing its role and substantial work has been done post-Nichols," said former Agency procurement director Steve Rowsell.
But another highways expert told NCE that there were also some very good candidates from the private sector who would fit the bill for the £180,000-a-year role, with Gifford director David Tarrant among the names mentioned.
All agreed that it would need to be someone with the political skills to interface between the Agency and the Department for Transport.
"The thing previous chief executives have found most challenging is dealing with the interface associated with being a chief executive of the agency, but also sitting on the main board of DfT," said Rowsell.
The Agency said this week it was looking for someone with "an excellent track record in leading large-scale operations" and with experience in steering through "change programmes".
The post is being offered as a three-year fixed-term appointment with the "possibility of permanency".
The prime internal candidates may even get a taste of the job they might be applying for as the Agency said this week that it would not rush the appointment and was ready to make an interim appointment.