1. Let' the road.

2. Let' the trail.
3. Let' the ______.
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Is #2 used frequently? Can we use other nouns in the blank in #3, which have the same meaning as #1 or #2?

They are similar.

I'd use #1 when getting ready to go for a ride in a car, whether for a short or long trip. Or maybe I'd use when telling someone else to get going!
https://www.youtube.com/watch?v=i8DRen60X10
Here it is in a Ray Charles song!!

I'd use #2 if I were getting ready to go on a hike in the hills or mountains or something like that.

1. The phrase "Let' the road" is a common expression that means to start a journey or go on a trip, typically by driving. It is often used to indicate a sense of excitement or readiness to embark on an adventure.

2. "Let' the trail" is less common than "Let' the road," but it is still a recognizable phrase. "Hitting the trail" refers to going hiking or embarking on a journey by foot, typically on a hiking trail or a path through nature. It implies a sense of venturing into the outdoors or exploring.

3. In the blank space for #3, you can use various other nouns to convey a similar meaning to #1 or #2, depending on the context and activity you have in mind. Some examples could be:

- "Let' the slopes": This phrase is often used when referring to skiing or snowboarding, and it indicates a desire to go downhill on a mountain slope.

- "Let' the water": This phrase can be used when going swimming, diving, sailing, or participating in any water-related activity.

- "Let' the gym": This expression is commonly used when someone wants to go to the gym to exercise or work out.

- "Let' the field/court": This phrase suggests participating in a sports activity, such as soccer, basketball, or any other sport played on a field or a court.

Ultimately, the noun you choose for #3 will depend on what specific activity or location you want to emphasize.