Likely will just want to grind this out, as there really isn't an incentive to honk the horn (at least in my opinion).
Therefore, I did the following:
1) Find a safe place to stop
2) Start clicking the Left Stick Button
3) Repeat step 2 until the achievement unlocks
I didn't monitor the status of the achievement and just started watching TV while clicking. Checked back after a little while and had the achievement. If you want to listen and monitor, consider selecting a horn that doesn't bother you if you have on constant, or mute your TV during the grind.