The low speed is said to be to protect road workers because they will be working so close to traffic.
It will be enforced by convoy vehicles leading motorists through the roadworks.
The restrictions cover four stretches between Perth and Inverness being repaired over six weeks from Monday.
Three of the sites are on normally 70mph southbound dual carriageways. Northbound traffic is unaffected.
The £1.2 million work starts on a half mile of dual carriageway near Drumochter, over two weeks from Monday.
BEAR Scotland, which maintains the trunk road for the Scottish Government’s Transport Scotland agency, said it would be confined to weekdays to minimise disruption.
Neil Greig, policy and research director of motoring group IAM RoadSmart, said “keeping traffic moving safely” had to be the top priotity.
“Transport Scotland should continually monitor and review the operation to ensure delays are not excessive.
“Accurate information will also be crucial, so additional electronic signs should also be provided.”
Convoys will operate on the one of the southbound lanes from 7am to 7pm while the other is resurfaced.
Overnight resurfacing of a half-mile stretch of single carriageway at Newmill, four miles north of Perth, follows.
It will happen from 7pm to 7am from Sunday, 19 June for two weeks, except on Friday and Saturday nights.
A 30mph speed limit will be in force during the day.
Convoys will also be used over a quarter-mile of dual carriageway at Ballinluig, south of Pitlochry, from 7am to 7pm for two weeks from Monday, 27 June.
The final work is at Badacreamh, eight miles south of Inverness, for two weeks from Monday, 11 July.
Similar restrictions will operate over a quarter-mile section of the southbound dual carriageway from 7am to 7pm.
Bear Scotland spokesman Eddie Ross said the work would “greatly improve the condition and safety of the route at these areas”.
He said: “We’ve taken steps to minimise disruption as much as possible by avoiding working at weekends at all four sites, and carrying out work overnight at Newmill, where the route is single carriageway.”