Deceleration on the highway

Would you care to be a bit more specific? How high the speed? Hills? Very heavy traffic?

Prii are designed to roll, coast, glide, cut through the wind more efficiently than most other cars. This is a good thing for gas mileage. When you lift your foot off the accelerator, the car doesn't decelerate as rapidly as most. It is designed in, purposefully.

Keep a 2 or 3 second cushion between you and the vehicle in front of you, and you should be happy. I realize that invites careless drivers to cut in front of you, but them's the brakes.

If you are in hilly terrain with steep declines, there is the B selection on the shifter to act as an engine brake and save the mechanical brakes from overheating.

 

You may already know, but it should be noted that going 75-80 mph you'll get maybe 43-47 mpg.

The car will decelerate fairly quickly at those higher speeds due to air resistance, too.

 

given:

gliding = slight throttle to remove regen (or in neutral)
coasting = no throttle or brake input

then:

gliding is better than coasting
coasting is better than adding more regen from the brake pedal
regen braking is better than friction braking
friction braking is better than hitting something.

Your brakes will slow the car as quickly as those of any other car, more or less.
Leave more distance from the car ahead so that you need the brakes less.

friction braking converts your momentum to waste heat.
regen braking converts your momentum to electricity in the HV pack, but incurs conversion losses.