Roundabout

You can easily get this achievement in about a minute if you remap your buttons. Go to Xbox settings -- kinect and devices -- devices and accessories. Click configure and then button mapping. I remapped the left click to the x button (don't use a or b or else navigating gets difficult).

You can turn on the achievement tracker if you want, but either way go back into the game and just mash the x button. I use two hands like I'm drumming on it. Once the achievement pops, go back into the settings and click "restore original."

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.