Building a new wash (even express wash) will not create new demand because all the motorists who will buy from new wash are already buying all they need elsewhere.
For example, start with planning a new wash.
Assume traffic count is 12,500 and expected capture of 0.004. The developer anticipates sales volumes of 50 cars a day or peak demand of 12 cars an hour. So, the developer decides to build one in-bay. However, after 2-years, volume is 38 cars a day or effective capture of 0.003.
Failing to make projections, information (relationship between traffic count and sales volume), suggests no unmet demand to fulfill. Otherwise, sales volume would be higher. In other words, there is no public need for another in-bay.
On the other hand, if volume year two is 75 cars a day (effective capture of 0.006 and average of 6 cars an hour or peak demand of 18 cars an hour), we would arrive at another conclusion.
For example, if in-bay is capable of producing 12 cars an hour, an arrival rate of 6 cars an hour would not lead to long waiting lines.
At arrival rate of 9 cars an hour, there would be 3 to 4 cars in line 40% of the time. However, at arrival rate of 18 cars an hour (peak conditions), the waiting ling would grow indefinitely (exceeding long). As a result, potential cannot be reached.
This would provide some evidence for adding a second in-bay unit.
Enticing wand-bay users to cross-over would require the same type of market intervention that causes customers to abandon self-service and full-service locations for express exterior carwash locations.
Retailers looking to do this try to create a sustainable competitive advantage by means of cost, differentiation or niche strategy. In other words, you have to give your wand-bay customers some good reason(s) to cross-over to in-bay.
For example, is the price better? Is it more convenient (faster)? Is the quality better? Is it safer?