A well-functioning Wal-Mart should have a low portion of occupied parking space. It should also have few people in line. when Wal-Mart had their bad spell a few years ago, I sent them a note telling them this.  There are two ways to make money at Wal-Mart. One is customer through-put. In contrast to Wal-Mart’s obvious ignorance, people do not like waiting in line. A cashier might cost Wal-Mart $12/hour, yet that cashier has the potential to collect hundreds per hour.  When we see those lines, we do not shop casually, and are not lured into spontaneously picking up unplanned purchases. Instead, we strive to grab the essentials, and hurry out.  When we pass by and see a totally full parking lot, we KNOW the check-out lines are worse than usual. So, we pass, and plan to make the trip later. I myself prefer grabbing a $4 gallon of milk at the convenience store because check-out is ALWAYS quick and I ALWAYS park near the door.  Wal-Mart COULD achieve that SAME phenomenon SIMPLY by paying what it takes to get an army of cashiers.  But, like most retailers, they prefer to “cut costs.”  What I am proposing can empirically be assessed. Average sale, customers checked-out per hour, portion of customers with impulse/promo items in their order, and wage percent: numerator of wage dollars to denominator of total sales.  Throw in: portion of parking lot occupied and you will see that, at volume times, a better performing store shows a smaller ratio of parking lot volume (num) to total sales (denom).  Oh – if that is not good enough, just ask the customers. Or, look at Target: never much of a wait, and they always seem to have parking closer to door than Wal-Mart.  It does not pay to be stingy with that $7/hour cashier.  What to do if the cashiers do not have customers to serve? 1. clean up; 2: ask who wants to go home early. Problem sloved.
A Chicago company called Remote Sensing Metrics LLC has been using satellite images to track the number of cars in Wal-Mart parking lots, as a means of helping forecast earnings at the retail chain.