Traction Control

Well i dont have a button to turn mine off, but I would do so on bumpy city roads where the traction control may be easily triggered. From what I have heard it doesn't help much in any situation. I would leave it on if you go up to the mountains to ski though.

 

More than likely when stuck in mud or snow.
In other words, times when you wish the wheels to spin.

 

Traction control does for "going" what anti-lock brakes do for "stopping". If one drive wheel spins more than the other then it cuts power for a moment to stop the slipping and then eases it back on again. It can be very frustrating in snowy conditions where you need to, say, rock the car back and forth in order to get some momentum going.

 

A couple things:

I'm not sure whether we're thinking of the same thing here ... there is a widely discussed (on PriusChat) feature (of all Prius generations) where a road bump while lightly braking can cause a non-seamless transition from regen to antilock brakes ... but that's an effect of the ABS system, not TRAC, so turning TRAC off won't stop that from happening (nor would you really want to; the transition does disconcert people who aren't used to it, but the car really is doing what you would want it to, switching from two-wheel to four-wheel braking at the first sign of a possible grip issue).

For the Gen 1 Prius (2001 - 2003), that would be a complete description. The Gen 1 really only had a feature to dial back the power when wheels spun, and it was really there to protect the transaxle more than anything else, but they tried to spin it as a traction control feature.

Starting with Gen 2 in 2004, and ever since, it is a real traction control; it does not just blindly dial back power output, it will detect which wheel is slipping more and apply the hydraulic brake to that wheel, so that the slipping wheel is slowed, and power that was being wasted at that wheel is diverted to the wheel that has more grip.

This explanation is from page TH-47 of the 2004 (Gen 2) New Car Features Manual:


You can see that the system is designed to allow some wheel slip: the drive wheel speed is allowed to stay slightly above the true vehicle speed, which can be helpful on some surfaces like snow, but it doesn't allow the kind of excessive spin that just wastes all the traction you've got.

If you set out to make a video demonstrating exactly what that graph shows, you'd end up with one just like this one here:

... where the driver is giving a steady go-pedal input, the car is continuing to pull with the wheels, but always making sure not to spin excessively and destroy traction.

-Chap