Almost all of the TT course will be open from 5pm till 6pm on race days for ordinary traffic to move around the island.
Highways bosses have made the decision after complaints of congestion in previous years.
It applies to the whole course, except for the mountain section between Barrule Park, Ramsey and Johnny Watterson’s Lane, Onchan.
A Department of Infrastructure spokesman said: ‘This measure will give the public more certainty about when they will be able to travel and allow people living on or inside the course to get home.
‘This will apply on every race day or postponed race day with the exception of Senior Race day (Friday).
‘Drivers should be aware that access will not be available to the Creg-ny-Baa except via the Creg-ny-Baa Back Road.’
In addition normal crossing points and access corridors will be opened during intervals between races.
These crossing points and access corridors are at:-
Douglas: St Ninian’s crossroads (Ballaquayle Road to Ballanard Road)
Douglas: Bottom of Bray Hill (Stoney Road to Tromode Road)
A1, Douglas and Braddan: Between Quarterbridge and Strang Road, Union Mills;
A1 (Peel to Douglas main road): Between Crosby crossroads and Ballavitchel Road;
A3: Barregarrow crossroads, Michael;
A3: Kirk Michael, between Douglas Road Corner and the entrance to Faaie-ny-Cabbal estate
A3: Ballaugh crossroads (Station Road to Glen Road, Ballaugh)
A3: Sulby Glen crossroads, Lezayre;
Ramsey Bus Station: (Christian Street to Westbourne Road)
Ramsey: Between Lheaney Road and Barrule Park;
A18, Onchan and Douglas: Between Signpost Corner and Bedstead Corner;
Onchan and Douglas: Between Governor’s Road and Second Avenue on Glencrutchery Road.
On Senior Race Day the roads will close earlier than usual with the mountain road closing at 9am and the rest of the course at 9:30am.
This will allow the race programme to get started earlier to give a better chance of all events being completed before 5pm.
However, if racing is delayed until later in the day it cannot be guaranteed that the course will be opened up between 5pm and 6pm and announcements will be made confirming arrangements.
A DoI spokesman said: ‘The department hopes that the public will benefit in having clear road opening times most days with a more flexible approach to Senior Race day when disruption to traffic is likely to be less as it is a public holiday.’