30km/h on all track

Is there a way I can suddenly make the tracks speed limit to 30 when ever I feel like it in driver mode? If the limit is 60 but I can suddenly make all the track limit to 30 without going into surveyor and starting the session over again
I don't think it's possible to change the track speed as such, but you can limit the speed of individual locos with the command 'Set Max Speed To' which will have the same effect.