Tank Traps... They should just throw tank traps out here.
T4 here is a hard issue as its part of the other layout.
If you want to use sensors how exactly do you do that, as something needs to be embeded in the track for the car to read?
If its passive how do you keep the car from registering it all the time when using the alternate layout?
Having an active sensor in the track is an upkeep and installation nightmare, especially if you are having to install with a switched configuration where there are multiple things to go wrong.
The sample rate on the on-car sensor will have to be very high/sensitive to stop false positives, as well as having to be combined with some other system to understand the location of the car on the layout along with the sensor data to prevent false positives on different configurations.
Can it be done.. yes, but its expensive, maintenance heavy and not fool proof.
Its not like race control lacks the cameras and eyes to do the same and save on the money/complexity.
You can't really do anything to the off track area as its "on track" for the other layout use.
This really only leaves doing something to the fully off track area where people are re-joining the layout used for the race.
Don't really want to put anything slick here that would toss a car out of control back on the track, nor a big bump as that would do the same.
The small amount of area there means even the most abrasive paint wouldn't likely detract alot.
KISS just points to having race control just do their job and follow the rules. Give penalties for cars running off the track t consistently. All the rest of the solutions are just trying to take the problem out of race controls hands where its their job in the 1st place.