To stop the traffic, it would be very suitable. Who can give a link to the siren with a Manual? To make it more realistic.

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Not sure if you're looking for an actual siren, or just a way to control it manually, but if it's the 2nd option, ELS has a key for manual siren. I think it's T, but I'm not sure as I don't use ELS a lot.

So far, there's not much of a way to control a wail tone manually, the closest you will get to the effect, is using the ELS V7-V8 feature.

What you are looking for is Emergency Lighting System, also known as ELS. I would say use that, if you are using ELS cars (otherwise it will not work) but when you are using it in game just press "T" for the Manuel siren.

There's also a (hidden) feature in LCPDFR for non-ELS vehicles. If you activate the lights only mode with / on the numpad, you can then hold down that same key for a manual siren similar to ELS (hold it for a few seconds for it to start). I think you can even press your default GTA IV horn button along with / to use an alternate siren tone.

 

In addition to that, after pressing your default GTA IV horn key to activate the siren, you can press * on the numpad to toggle lights only mode (you might first have to go through the / stages once, something I noticed with EFLC cars specifically). Using this, you can repeatedly press * for a manual-like siren.

What you are looking for is Emergency Lighting System, also known as ELS. I would say use that, if you are using ELS cars (otherwise it will not work) but when you are using it in game just press "T" for the Manuel siren.

 

I use the ELS and use the "T", but the sound is not very realistic.

I saw the special sounds, like this: https://www.youtube.com/watch?v=gi4iZM5qMto