My Prius - One week of ownership and lessons learned so far

I believe its direct mechanical link from ICE to wheels is more intended for high speed driving on level roads, not the lower speeds typical of climbing hills.

 

Mostly power splitting will occur in relatively steady state cruising above 40 mph or so and isn't very obvious to the operator.
With the latest models, there's the flexibility of opting to run the ICE to maintain a given state of charge. In all models there's a Mountain Mode that will maintain or build up a buffer of battery charge in anticipation of long climbs (they suggest kicking it in 20 minutes or more before the anticipated climb).
The most noticeable thing about the Volt is that the driving experience (accelerator response, etc.) are essentially the same whether in electric (charge depletion) mode hold (charge sustaining) mode or battery depleted (range extending) mode. It feels like a smooth, torquey electric car in all modes. The only exception will be if you deplete the battery sufficiently in a mountain pass that power gets limited to what the engine (a 1.4 liter) can put out, derated for inefficiencies.

 

Looks like you're correct. I could not find any regulations as I had thought. However I found plenty of references which indicate that the minimum speed is a mechanical issue. Several reasons were given such as inaccuracies at low speed as well as insufficient vacuum pressure needed to maintain low speeds.

 

There could be vacuum issues at low speeds in an ICE car, but the Prius's throttle is fly-by-wire. The ECU should be able to control the car down to zero. It requires a lot more software/hardware that Toyota apparently neglected to include, presumably for cost reasons. And it would have cost us as consumers a great deal more.