Wikipedia defines dark pattern “a user interface that has been carefully crafted to trick users into doing things”.
Then, “carefully crafting fake notifications about customers renting cars” clearly falls into the same category.
This is what rentalcars.com does. Once you select a city and some dates, you’ll get the list of cars available, and something else: a set of notifications that spawn with a random interval from one another implying other customers are renting cars in the same city you’re looking for a car to rent.
Does this put some pressure on you, hastening you into buying a car before all disappear? Yes.
Should you trust it? Actually, no.
If you inspect the source code, this is what you’ll see (click to view it full size):
Have a look yourself and you’ll find that as the page loads, a function will set to display, in the top right corner, some “pre-cooked” notification, lasting for 4.8 seconds, popping at random intervals from one another.
Reload the same page a few times and you’ll see those 4 notifications pop up time and again.
Sure it can’t be that legal, can it?