An interesting anonymization - useless for instant traffic updates, though - would be to fuzz the location based on vehicle speed. When you're moving quickly, very precise data would be shared, but when you're stopped, uncertainty would be increased - that way, where you're parked every night would be much less precise than the highway you take to work.

This wouldn't do much good in more rural areas, of course - you could probably zero in on exactly where my parents park their car with a month's worth of data even if you add a mile-wide fuzz to every parked check-in.