Here are some test results for magnetic fields in some popular cars in Sweden from 2002 (including the Prius I):
Magnetic fields
The numbers are in microtesla which is used to measure magnetic fields. The left values are at standstill with engine idling, the right when riding in 90 km/h (about 60 miles per hour?). As you can see the Prius has low values. I am not sure how this translates to EMF and milligauss. Maybe someone can explain?
Some translation for the numbers:
Vänster fot - Left foot
Höger fot - Right foot
Golv - Floor
Tröskel - Threshold?
Säte - Seat
Brösthöjd - Chest height
Huvudhöjd - Head height