(CNN)UPS trucks almost never take left-hand turns.
By favoring right-hand turns at all times -- unless a left is unavoidable -- the carrier saves millions of gallons of fuel each year, and avoids emissions equivalent to over 20,000 passenger cars.
The practice started decades ago, before computers and GPS, and is now managed by a software that conjures the most efficient route for each truck.
Left-hand turns are generally considered unsafe and wasteful on right-hand driving roads, such as those in the US.
"Left-turning traffic typically has to turn against a flow of oncoming vehicles," explains Tom Vanderbilt, author of the book "Traffic: Why we drive the way we do."
"This can not only be dangerous, but makes traffic build up, unless you install a dedicated left-turn 'phase,' which is fine but basically adds 30 or 45 seconds to everyone else's single time," he said.
A study on crash factors in intersection-related accidents from the US National Highway Traffic Safety Association shows that turning left is one of the leading "critical pre-crash events" (an event that made a collision inevitable), occurring in 22.2 percent of crashes, as opposed to 1.2 percent for right turns. About 61 percent of crashes that occur while turning or crossing an intersection involve left turns, as opposed to just 3.1 percent involving right turns.
Left turns are also three times more likely to kill pedestrians than right ones, according to data collected by New York City's transportation planners.
"A left-hand turn is also less fuel efficient," said Jack Levis, UPS Senior Director of Process Management, "because your car's idling longer, which is also not good for your vehicle."
UPS does not ban left turns outright, says Levis: "We will make left hand turns, but not ones that are unnecessary. We don't need to go in circles all day long by making only right hand turns. We have tools analyze the number of left hand turns for each route, and we can work out which ones are avoidable."
The procedure is now incorporated in most countries around the world.
To this end, the carrier created its own maps, which it says are more accurate that commercially available ones: "We can differentiate more important left-hand turns from unimportant ones. Google Maps has no concept of not making a left-hand turn, it just shows the most direct way to reach your destination. We have the ability to penalize some of those," Levis told CNN.
The system knows about parking lots, private driveways, variable speed limits and roads that are inaccessible for a truck.
The software can give an undesirable left turn a penalty that adds 20 seconds to the estimated route time. In that case, going around the block and turning always right might offset that 20-second penalty: "We were able to turn off left hand turns," said Levis.
Taking a longer route while still saving time and fuel might sound confusing, even to UPS drivers, according to Vanderbilt: "I've actually been to UPS's logistics center and discussed this with their lead engineers," he said.
"A lot of individual drivers felt the new routing software was making their trips longer, but they were later proven wrong. This is the thing about traffic, it's such a complex system that often the individual cannot get a sense of the overall efficiency of the system, and optimize accordingly. It's also one of the counterintuitive, 'slower-is-faster' effects you often see in traffic."
The rule, says Levis, can also be applied to left-hand driving countries, such as Australia and the UK, where it discourages right-hand turns.
In 2008, it launched a routing software to calculate the best possible route for each truck while favoring right-hand turns, called Orion: "It took 10 years to get it right. The hardest part was making it think more like a driver and less like a computer," said Levis.
UPS, which makes 18 million deliveries a day in the US, says that Orion analyzes 250 million address points a day and performs 30,000 route optimizations per minute. This saves the company $300 to $400 million annually in fuel, wages and vehicle running costs: "Our basic routines were already good, and allowed us to save about 85 million miles a year. When we put Orion on top of those, it shaved off an extra 100 million miles, and the savings got up to 185 million miles a year."
"This no left hand turn discussion came out of a conversation about what regular consumers can do to be more fuel efficient," explains Levis.
Examples would be consolidating trips, parking in a central location to walk to several nearby destinations, and using the right vehicle for the job.
But would an avoidance of left-hand turns help save fuel, or time, in everyday driving?
"The method works well for pre-planned deliveries, but in daily driving, our routes are usually far less random," said Wayne Gerdes, who holds world records for fuel-efficient driving and is an advocate of a technique called "Hypermiling."
Most of us are not going to multiple different addresses, but faced with the challenge of optimizing a daily commute that may not have changed for years, says Gerdes: "Eliminating left turns cannot in many cases be incorporated into our own more ridged daily routing, so my suggestion is to map multiple routes and work them into your commute.
"Do not stop the optimization as traffic, roads and conditions change year over year for many of us. If one route has a new roundabout for example, try it! When I experience a new roundabout on a road I have discarded for decades because of stop signs or non-priority lights, I will prioritize this new route over others, while saving both fuel and time.