United Parcel Service (UPS) celebrates its 110th anniversary this year, but it’s not likely any of their drivers are going to stop for cake. With 4.9 billion packages routed via UPS trucks every year, their priority is getting your goods to your front door while maximizing productivity.

In their pursuit of expediency, in the 1970s UPS began mandating that its fleet of brown-clothed drivers avoid turning left whenever possible. While that might seem like a curious policy, it turns out that it’s the most economical way to distribute packages.

When motor vehicles turn left, they’re operating against the flow of traffic. In addition to creating safety issues with collisions, it also means a truck—say, one full of Amazon packages—is going to sit idling waiting for cross-traffic to abate before it can successfully navigate away from it, wasting both time and fuel.

To combat this, UPS uses an onboard navigational system they’ve dubbed ORION (On-Road Integrated Optimization and Navigation) to tell drivers the best route to take that’s made up primarily of right turns. By looping right during delivery runs, the company estimates it saves up to 10 million gallons of fuel every year and shortens paths by an average of six to eight miles.

Not all left turns are avoidable—UPS figures drivers go left about 10 percent of the time, or more if they’re in more rural areas. But the general rule of thumb is to keep right to keep on schedule.

Would that strategy work for all drivers? Not necessarily. According to traffic analysts, commuters and other non-commercial drivers are typically using the same route, minimizing the necessity to cut corners.

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