The speed limit on the A38 Saltash Bypass between the Carkeel Roundabout and the Saltash Tunnel is being cut from 70mph to 50mph.
National Highways has announced the speed reduction saying it is being introduced as “part of its commitment to safety and air quality”.
There have been calls made in the past for the speed limit to be reduced on this section of the bypass which opened more than three decades ago.
The bypass to the west of the Carkeel Roundabout has a 40mph speed limit. In addition, average speed cameras are being installed to enforce the 30mph limit through the tunnel and on the B3271 North Road diversion route.
National Highways tunnel manager Julian Mitchell said: “The speed reduction measures are much needed.
“This section of the A38 sees a number of minor collisions as motorists try to reduce speed quickly on the approach to the tunnel and the Carkeel Roundabout, and we are aware of vehicles exceeding the speed limit along the B3271 North Road diversion route when we close the tunnel.”
The announcement comes as National Highways prepares for a phased programme of safety, refurbishment and modernisation work in and around the tunnel.
The 35-year-old tunnel is set for a £20m upgrade, with the bulk of the work taking place next year.
Site preparation work, including the removal of old cabling, will take place ahead of and alongside the speed reduction scheme.
Mr Mitchell said: “The tunnel is safe but it is essential that we replace our systems which have been operating for more than 20 years.
“We’re currently in the detailed design and construction planning stages, and, in the meantime, while the speed reduction scheme takes place, we’ll be preparing the site by stripping out the outdated and redundant equipment and cleaning and preparing cable routes for the kilometres of cabling that will need to be installed. There’s been a considerable amount of planning over several years to identify efficiencies and reduce the number of future maintenance closures and once we have a timeframe for next year’s upgrade work, we’ll communicate that in advance and will hold stakeholder and public open days to provide further information.”
To enable the modernisation preparation work, the tunnel will be closed on weeknights, from 8pm to 6am, between Monday, September 18, and October 7, with traffic diverted via the B3271 North Road route.
For the speed reduction scheme, the tunnel and A38 to the Carkeel Roundabout will be closed overnight, on weeknights only, between October 2 and October 24.
The overnight closures will be in place from 8pm to 6am, and from October 25 to November 7 two-way traffic signals will be in place overnight on the B3271 North Road route.
There will be more weeknight overnight closures of the tunnel and the A38 up to the roundabout in place between November 8 and December 20, with two-way traffic signals continuing along the B3271 North Road route overnight only.
Mr Mitchell said: “We appreciate that the schemes will impact on road users and Saltash residents, but we’re keeping the closures to an absolute minimum and we thank all drivers and residents in advance for their co-operation and patience while we carry out this essential safety work.
“Being a bi-directional tube with three lanes and tidal flow, and connecting to the Tamar Bridge, the tunnel is unique, and it’s difficult to work safely within the structure without closing it overnight. We apologise in advance for any inconvenience and advise people to plan ahead and allow a little extra time for their overnight journeys.”
During the work, drivers can obtain travel information via www.trafficengland.com, the @HighwaysSWEST Twitter feed or by phoning the National Highways Information Line on 0300 123 5000.