Big,
Back in 2005 I added a 2nd IBA at my fist site, one year after the initial build. I too watched long lines, cars driving off, and lived in fear of a Friday afternoon breakdown that I couldnt fix. The incremental volume amounted to +30% of the single IBA in the first year. It continued to grow after that and they now wash the same amount of cars in each bay(+/- 3%) and the total is probably 40% over the initial year. Some of that growth is the amount of time it takes to buildup customers at a new site. Some of it is definitely adding a 2nd bay. I think alot of the drive offs have to do with the number of options customers have (ie. proximity of competitors).
When adding another IBA, I would highly recommend looking at the Tandem, or another soft touch machine. I have a 3yr old site with 2-LW360's and 3 SS bays. In April of this year we converted one of the SS bays and installed a Tandem. So far it's washing as many cars as the two L360's combined on a daily basis. I know it won't be able to hold those numbers when the banner wash days are upon us, but I have been amazed at the positive feedback from customers. My
PDQ distributor tells me they are seeing more and more sites with a touch and touch free installed next to each other. It not only broadens your product offering and allows you to attract another segment of customers, it also gives your current customer base more options for getting a clean car.