I am a Campbell resident for 2 years now. My wife and I would love to have an In-n-Out Burger restaurant in Campbell. Occasionally, we traveled to other neighbor towns to visit this restaurant, and having one close by would be awesome.
The restaurant will definitely be an attraction for many people, both Campbell residents and non-residents. There are many other fast-food restaurants in the areas, some of which are good, some not so good. In-n-Out is known for its better-quality products, that's why it is so popular when comparing it to other fast-food chains.
Also, I am sure the new restaurant will will create new jobs for Campbell residents too, which is another great advantage.
As for the traffic problems... Cars go in in and out from the "drive through" area slowly. I've never seen a ton of cars entering a drive though of a restaurant, or causing traffic congestion. Cars are slow when going in, or out, especially that they will have to yield to all traffic going on the main road.
Finally, there already was a restaurant in the place where In-n-Out restaurant is planning to be built. Did that restaurant cause major traffic issues to the area? No, it didn't. Yes, In-n-Out may get more visitors. Anyway, I don't think it will have such a major impact on traffic, to prohibit the restaurant from being built there.