Natural gas is invisible. Guess what isn't?

In Dublin, the operators of our light tram system (the Luas) were forced to add bells after several collisions and near misses with drivers and pedestrians.

Cars are smaller than trains.

Here in the SF Bay Area, we routinely have collisions between our (loud, and belled) train and drivers and pedestrians. It turns out, sometimes people run into trains. Even noisy ones.

You can only see cars when you're facing in the right direction and have a direct line of sight. Sure, if we redesigned every street on the planet to have perfect visibility with no blind corners, there'd be little need to worry. This may involve demolishing a few million towns and cities though.

They're both invisible to a blind person.

So are knives, a clearly dangerous object! Knives should be regulated to emit constant noise.

No, seriously, I think you do the blind a disservice. I don't think that blind people rely on, "Hey, did I hear something" to mean, "I can blithely cross the street without danger."

What a childish analogy, knives are not moving at high speed on every street.

I see a market for car engine "ringtones."

Please, anything but this.

We have to do everything in our power to make sure cars don't have "ringtones".