For outdoor advertising, what is more important - good copy or good visual?

Good visual of course and keep your copy for a minimum and sometimes no words.
Remember Less is more.
Plus simplicity is more classy.
Driving people don't have the time to look and read the ads.
But you should focus on making a memorable visual that would get stuck in the public's unconscious even if they did not firmly look at it.

for outdoor advertising good visual is more important thing because if the visual is strong and attractive people will notice it once and would like to watch the advertisement and then what the advertisement want to spread there message comes next

One of the most important role for outdoor advertising that you have only 3 seconds to deliver your massage.
As Ahmad said, driving people have no time to look .
Your artwork must be impact and simple.
Don’t use a lot of visual elements, don’t crowd it.
add the slogan of the camping, in an easy eye catching way and readable big size.

I would say a good concept.
Any ad, whether outdoor, print, online, is just a combination of copy and visual.
Some ads don't have copy because the visual is strong enough to convey the message; and some ads do away with visual because the copy is treated as a visual.
There is no hard and fast rule that either one should come first.
But for outdoor, unless your copywriter can keep his words under 5, I'd suggest you start working on a good visual.

The most important is to have a powerful, clear and impactful message, whether you convey it with copy or visual

For successful transit advertising, a powerful visual and crsip but very short copy are the essentials. Remember, transit advertising (outdoor iis an old term) means your target groupps are moving, they are in transition and don't have the luxury of more than a few "safe" seconds to spare tto see your billboard.  

for outdoor ad. should be in 2 or 3 colors combinations. & eye catching cartoons or wordings.

Both and mixed with good concept and ideas of advertising

I think in this particular comparison, the visual is the most important But if we look deeper more ...
We can find everything related to the first and second item of the Law of AIDA Attention and Interest) ..
More important at different levels)

Both are needed to attract the eyes by your creative visual, and to contact with them by your copy.

Good visuals.

The advertisement should be well thought out and creatively presented. The words in the copy can actually hinder the message as more the words, the more types of interpretation. That can dilute the original essence of the message.

Words are important though, but not at the cost of overdoing it.