“How does the total demand shift when another store opens up? “Are….sales…. redistributed among the existing and new stores?”
When a new store enters the market, part of its success in reallocating demand inherent in a stream of traffic (retail gravitation) is the overall design objective.
For example, Checker’s, a limited quick serve restaurant (QSR) targets mostly young adults who want grease, quickly and cheap prices. Most sites are double drive-thru, no inside seating. Average service time is 45 seconds.
McDonald’s is a full-service QSR and targets include moms who eat salads with kids who eat half a happy meal and throw toy out the window. Newer sites have long narrow building for more drive thru action, smaller inside seating, all have bathrooms, some have playhouse. Average service time is about 3-minutes.
“Is there some kind of unmet demand in a market that an additional store serves?”
Yes, it’s possible.
For example, I know markets saturated with self-serve and in-bay at gas sites but no conveyors or assisted-service. Markets like these have unmet demand and are also candidates for assimilation.
“The IRS and BPI would be curious gages to understand but against what standard are they assessed to draw conclusions about in this specific market of car washes?”
One standard is per capita. For example, based on recent census data, per capita spending for conveyor segment is $46.00. In IRS, this gives measure of E. In BPI, it gives measure of RS.
Another standard is same store sales. Based on census data, same store sales for conveyor is $550,000. In IRS, this gives measure for S.
In BPI, per capita income ratios (area / national average) give a measure of I.
Another standard is having an analogue of stores to base conclusions on.