It sounds like cruise control to me.
Perhaps what you want is the ability to have an always "half on" cruise control. Half in the sense that it maintains current speed, but never accelerates above it.
In the Prius, and presumably other Toyota hybrids, after some practice you can hold the accelerator pedal at just the right position so that you don't use either battery power or engine power. The car slows down but very slowly. This is the most efficient way to drive the Prius because you don't throw away energy by regenerating*, but it takes quite a bit of practice to get it right. Additional instrumentation helps a lot.
* Regeneration in the Prius happens when you let your foot off the accelerator. It also happens when you apply the brakes. The amount of regeneration using only the accelerator is small. Toyota did this to mimic and automatic transmission however in doing so they made the Prius much harder to drive efficiently. Capturing some of the kinetic energy using regeneration is much better than throwing it all away in the form of heat the way old fashioned cars do, but it's not as efficient as using kinetic energy to glide because when gliding only a small amount of energy is lost to heat (in the tires and bearings).
W. Brown measured the regeneration that happens on the 2004 Prius:
WB> All of the following records are data captures that occurred from 50
WB> MPH to 10 MPH. Each record is the result of the average from 3
WB> separate runs:
WB>
WB> Mode.........Amps.......Seconds........Cumulative w/h
WB> Braking.......90...........9................54.03
WB> Braking.......70..........15................64.38
WB> Braking.......60..........19................71.01
WB> D.............18..........51................59.50
WB> B.............26..........30................48.35
WB>
WB> It is interesting that in this test, "B" is almost 19% less
WB> effective than "D."
WB>
WB> Of great interest however, is the "Braking @ 60 Amps" runs. This is
WB> by far the most efficient way to slow down a 2004 Prius. In fact it
WB> is almost 32% more efficient than "B."
WB>
WB> Here is a table that illustrates the several common ways we slow the
WB> Prius down and their ratings in efficiency percentages. "Braking @
WB> 60 Amps" is the most efficient way to slow the 2004 Prius down;
WB> therefore it is the 100% baseline or benchmark against which the
WB> others are compared:
WB>
WB> Mode.....................Efficiency
WB> Braking @ 60 Amps...........100%
WB> Braking @ 70 Amps............91%
WB> D............................84%
WB> Braking @ 90 Amps............76%
WB> B............................68%
WB> N.............................0%
, then the blue, then the dolphin gray with carbon gray wheels, then the brown, and then the multi-coat white. The green is too dark, IMO, and the standard white and black are just... "pedestrian" is the thought. The black looks good, but I've owned a black car and well, just... no.
