Police Sirens?

Its hard for the officer to drive fast while cranking on a mechanical siren.

If you were to mechanically link the engine to the siren, to avoid the driver having to crank, the gearing and on/off controllers would be heavy, prone to break down and much more difficult to replace or repair than the electronic models.

Electric sirens are smaller, lighter, and you can switch between a variety of tones. Mechanical ones are also more likely to break. 

I've always wondered that too and why can't they change the traffic signals like the Fire Trucks?