[enko]

I believe it's by design. Differing indicator frequencies make the signals more noticeable.

Back in the old days, turn signals were actuated by bimetallic thermal relays - a simple and highly reliable device. They were constructed with a range of frequencies by manufacturers and sold mixed together, so cars might receive the same part number but with an essentially random on/off period. This part would also mechanically create the familiar clicking sound of a turn signal actuating.

These days indicators are controlled electronically and I assume the randomness is programmed in.

[Consultant32452]

That's interesting and kinda funny since you still get the familiar clicking sound when your turn signal is on I can only assume that this sound is made artificially. It immediately reminded me of the fact that computer keyboards are naturally silent but they engineer in "clicky" sounds. This was very important in the old days when people were transitioning away from type writers and were accustomed to the audio feedback. Now we still have this, but keyboards are a lot quieter these days.

[ygra]

There are keyswitches that do click naturally, e.g. buckling spring keyswitches [1]. And it might be subjective, but I type faster with audible feedback, even though it can drive other people in the flat nuts.

[1] http://en.wikipedia.org/wiki/Buckling_spring