Korean humor

A few days ago my host mother tried a joke out on me as we were driving home from work.

When Korean school-children are asked what the colors on the traffic signal mean, they say that red means 'stop,' green means 'pass,' and yellow means--

Here my host mother stopped to see if I could anticipate the punchline, but with the slight language barrier I was a little slow on the uptake.

'accelerate.'