We hear it all the time; Customer Experience (CX) is the key to repeat business (AKA - understanding CX is key to your success.) As a result, most car wash operators do some form of surveys. A survey can be as simple as talking with your customers or as basic as a paper survey, but online surveys should be part of the mix in your overall survey strategy.
Why online? Paper surveys = time and money. Money to print. Your time to read. If you are going to use the survey information as a data point, someone's time to do data entry and to summarize the information collected. Online surveys do these tasks automatically. Online surveys represent a much more cost effective way to understand CX at your site(s).
Why online? More possibilities and better data. For example, maybe you've switched over to a new tire shine product. Online surveys & custom receipt messages would make it easy for you to ask every tire shine customer for input on tire shine quality after the change. With a paper survey, you could easily be asking non tire shine customers for input on a product they didn't receive (i.e.: you could actually be receiving feedback from a confused customer about wheel cleaning vs the tire shine product.)
Overall, online surveys don't replace the act of talking with your customers, they supplement it. It simply isn't possible to talk to every customer, but online surveys make it easier for you to hear the voices of those who you didn't speak with in person. Additionally, online surveys help you quantify the trends of your overall customer experience.