CHICAGO — A clean, well-lighted menu board. Speakers that clearly coo "may I take your order?" And the biggie: speed without screw-ups.

That is the gold standard for a fast-food drive-through, and McDonald's Corp. and its rivals have developed a science of sorts to try to meet it. Each year, the drive-through captures a little more of the fast-food industry's sales, so a smooth-running drive-through is a competitive advantage.

"People decide whether to come to your restaurant based on how long the drive-through line is," said R. Craig Coulter, chief scientist at HyperActive Technologies, a Pittsburgh-based restaurant software firm. Often, he said, "If there are seven cars in your line, they will go to your competitor."

More speed can translate into more sales. There's an industry maxim that for every seven-second reduction in drive-through service time, sales will increase 1 percent over time, said Matt Jennings, president of data management at Minnesota-based Restaurant Technologies.

But accuracy is most important to consumers, said Jennings and others in the industry.

Technomic Inc., a Chicago-based restaurant consultant, found in a survey last year that 31 percent of consumers said accuracy was the most important factor in getting takeout food. Speed was fifth, with 7 percent saying it was most important.

Most drive-through miscues stem from miscommunication between the order taker and the customer, industry experts said.

One of the big technological leaps of the past several years is aimed at cutting such miscommunication: the order confirmation board. At many chains, a customer's order will pop up on a screen at the menu board, allowing a visual check.

Other more recent innovations include digital audio for better speaker clarity and computer systems that generate rich data of drive-through sales patterns, allowing for restaurants to better adjust their inventories.

"There is an incredible amount of science in the drive-through," said Robert Marshall, McDonald's vice president of U.S. restaurant operations.

Drive-through efficiency is important enough that QSR, an industry publication, each fall ranks the best U.S. drive-throughs, and the fast-food business pays a lot of attention. This year's winner? Chick-fil-A, the chicken chain that has ranked first in five of the past six years. McDonald's came in second.

Drive-throughs make up about 70 percent of the industry's sales, up from about 60 percent six years ago, said Sherri Daye Scott, QSR's editor. The increase stems partly from a trend in recent years toward extended restaurant hours; late-night business is usually drive-through only.

But it's cultural, too. "I just think we are busier and we've become really accustomed to convenience," Scott said. "The idea of people not wanting to get out of their car has become so pervasive."

QSR commissions research of the nation's top 25 fast-food chains' drive-through operations, reviewing thousands of transactions. The publication looks at four factors: menu board appearance — clean and clear is good — speaker clarity, speed and accuracy. The last two are more heavily weighted than the first two.

Speed has improved over the years, Scott said, with this year's speediest, Wendy's, posting an average service time of 131 seconds. But accuracy fluctuates, Scott said. And improvements in speed often mean declines in accuracy.

"It's really difficult to find that sweet spot," she said.

At one McDonald's in Wheaton, Ill., a car pulls up to the menu board, tripping a sensor.

"Welcome to McDonald's. Would you like to try a mocha frappe?" the drive-through speaker chirps.

It's a recording of a store employee, not a live greeting. "You always get a consistent message that way," McDonald's executive Marshall said.

Inside the store on this morning, there's a crew of four devoted to the drive-through. One is staffing the register, a computer screen behind him showing pictures of cars. This is a double drive-through — two lanes — so cameras are constantly clicking and matching orders with cars to avoid mix-ups.

Another worker wearing a headset focuses on taking orders. Some fast-food chains have started using call centers to take orders; Marshall said McDonald's is in a "very early" call center test, at a small number of restaurants.

With a call center, a phone jockey in, say, Fargo, N.D., may take an order for a customer in Wheaton. The concept is in part aimed at freeing workers in the restaurant to focus on meal preparation and delivery.