(1) According to the meaning of the question: 74÷0.4= 185 (vehicle),

Then the frequency of speed 60 ~ 70 is185-(32+74+18+22) = 39, and the statistical chart is completed, as shown:

(2) The violation frequency is18+22185 = 0.216;

(3) The number of vehicles with a speed of 70-80 per hour corresponds to a fan-shaped central angle of 74 185× 360 = 144.

So the answer is: 144.