Civil engineering firm Balfour Beatty says it is essential smooth travel is maintained during the major project.
Transport Scotland said it had been alerted by the contractor for the A9 roadworks of an environmental incident.
The £185m contract forms part of a project to complete the dualling of the A9 from Inverness to Perth.
The measure will be introduced to assist with dualling the route between Inverness and Perth.
The reduction from 60 to 40mph is needed to protect road users and workers during an upgrade.
Transport Scotland said rescheduling the current timetable for the project would cause further delays.
The total cost of the dualling project was originally estimated to cost £197m.
It was previously hoped the six-mile stretch south of Inverness could be upgraded by the end of 2027.
Work on the Tomatin to Moy section had to be retendered last year after only one company showed interest.
