If the Interstate/US Route system had a motto/slogan...

[TravelingBethelite]

What would it be?

Mine would be a line from the classic "Take It Easy", by The Eagles, which in its whole, is a great metaphor for long-haul driving.

"Don't let the sound of your own wheels, drive you crazy..."

[NE2]

wat

[jeffandnicole]

I can't drive 55.

[cjk374]

"Welcome to Arkansas! If you think you are confused about why you are here, just try following some of our US routes by their signage."   

[AlexandriaVA]

Rambling Man

Convoy

Life in the Fast Lane

[noelbotevera]

Ugh. Pretty sure that describes interstates.

[kkt]

I dunno about everywhere, but in Washington it would be "expect delays"

[hbelkins]

The US route system's would probably be "No, you idiot, we aren't federal routes, and what the #$%& is viatology anyway?"

[SteveG1988]

Take the long road home

[The Nature Boy]

I don't know about the whole system but Interstate 95 could easily be "Main Street USA."

[Chris19001]

It's the best you're going to get, as we could never do it again...

[Jardine]

"Be prepared to stop"

[Buffaboy]

Fast or slow, it's the way to go.

[1995hoo]

Regarding the Interstates, there's always what Charles Kuralt said: "(I)t is now possible to travel from coast to coast without seeing anything."

(Sure, that's sometimes unfair, but when I want to focus on scenery rather than speed I almost always opt for the non-Interstate option unless I know it'll be excuciatingly slow or boring.)

[Rothman]

21st Century Schizoid Man

[Bruce]

For urban areas: Why bother?

For rural areas: Expect nothingness.

[wanderer2575]

Not exactly a slogan, but I always liked this exchange:

"Road, (n.)  A strip of land along which one may pass from where it is too tiresome to be to where it is futile to go."
(Ambrose Bierce, The Devil's Dictionary)

"Bierce was right about the futility of the destination, but he did not take into account the joy of the trip."
(Charles Kuralt, A Life on the Road)