Tires for grooved roads (grooved cement roads)

isn't that part of the freeway that has those grooves in the road?

 

You might get your alignment checked. On RWD cars, a little front end toe-in will increase stability. Because the Prius is FWD, you might need some combination of toe and caster to remedy this problem. Call an alignment shop and see what they recommend.

 

A little sway from grooves in the road shouldn't bother you... Consider yourself lucky to be able to drive on such nice roads! I always get great gas mileage when I'm on that stretch, as well as all the other stretches in SF Bay Area and Sacramento valley. Once I head further north all that rain and snow means the roads wear out quicker and they're way more rough and my MPG goes down. I'd easily trade the grooves on CA-85 over all the garbage I drive on in Oregon and Washington.

 

This isn't a very helpful response. The OP is obviously troubled by the wandering his/her car is exhibiting.

I took our other Toyota (MR2 Spyder) to a performance alignment shop after my fiancee mentioned she was scared by its tendency to wander. They gave it some front toe-in, which reduced the wandering, but didn't cure the problem.

In some cars, this just can't be cured without replacing the steering rack or radically altering the front suspension. We must choose a compromise between steering responsiveness / "feel" and high speed stability. The MR2 is a performance-oriented car and hence is very responsive. It can be a beast to handle in slippery conditions. The Prius shouldn't have such problems, and I've never experienced them in mine to the degree the MR2 exhibits.

 

By your own admissions only so much can be done... Other comments adress what can be done with alignment, I'm suggesting what can't be done... You're suggesting both... It takes lots of responses to make for helpful responses. I'm one of many no more or less helpful than you...