It happens all too frequently: You are stuck in your car in a traffic jam on I-395, Colesville Road or Kenilworth Avenue on the way to the courthouse, an important meeting or even a rendezvous with a long-lost lover.

The minutes tick away as the radio in the background reminds you that you have only a short time to get to your destination.

And you can't pull over to get to a pay telephone.

Recently a Texas oil company lost several million dollars on a spot market deal because an official driving through the state could not be reached to sign off on the transaction.

That company, like many others, desperately wants to upgrade the productivity of its employes by not losing touch with them while they are on the road.

Solutions available today are unsatisfactory. Beepers, for example, have a limited range. Most two-way car radios and similar systems have inherent problems such as the need to wait for a line -- even if complicated licensing procedures can be overcome. In New York City, to name just one, only a couple of dozen mobile telephone lines in cars can be in operation at one time.

Nationwide, there are only about 100,000 mobile telephones. And thousands of people across the country are waiting to get mobile, direct-dial telephones in their cars.

But for the past year, two projects have been under way that should alter dramatically the complex mobile telephone situation. Such projects could make mobile telephones widely available to businesses and others that for many years have looked for a way to break through a federal regulatory logjam and turn telephones in cars into an everyday part of U.S. life.

American Telephone & Telegraph Co. is involved in one of the projects, conducting an experiment in Chicago with something called a cellular telephone system.

Motorola Inc., already a leading manufacturer of mobile telephones, and American Radio Telephone Service Inc. (ARTS) of Baltimore are involved in the second project, an experiment in the Washington-Baltimore area with a similar, highly sophisticated type of automobile telephone.

At the same time, the Federal Communications Commission has lifted restrictions on dozens of other new frequencies that could make mobile telephones readily available to the public. Dozens more frequencies may be available by the end of the year.

Don't expect to see cheap mobile telephones in every showroom car soon, however. The equipment itself will cost $1,000 or more and the industry is not gearing up for a massive plunge at the broad consumer market.

But what is at stake is nothing less than a revolution in the way business is done in the United States.

Travis Marshall, a Washington-based vice president of Motorola, says the growth of such systems will "change a lot of habits and customs."

Devotees of mobile telephone systems say that ultimately they will add to business productivity and save energy. By being able to place and receive calls from a telephone in a car or even from a portable telephone anywhere, sales and other business people will be able to stay in constant touch with offices and clients. Gasoline consumption could be cut by eliminating the need for many trips.

In fact, AT&T marketing studies indicate that as much as 13 percent of the business community would be interested in purchasing one of these advanced systems.

"If all this happens, it's certainly going to dramatically improve one's ability to communicate," says Graham Randolph, president of ARTS.

The ARTS-Motorola program involves only employes of the two companies."This system will satisfy mobile communications needs through the end of the century," Randolph said.

"I can't help but feel that this is the most exciting thing that has happened in telecommunications in my 34 years in the business," says Louis Weinberg, the director of business marketing service development for AT&T, which is running an experiment with 2,000 persons selected at random in Chicago and has spent more than $100 million on cellular telephone system research and development.

Customers of that AT&T service pay $25 a month and 25 cents a minute for usage over 120 minutes. The average bill for the year-old experiment has been about $150 a month.

The FCC has sanctioned the experiments in the two cities for several reasons. Chicago is very flat, while Washington-Baltimore is hilly, presenting a different series of technological problems.

The cellular systems, which AT&T's Weinberg said will cost $10 million to $15 million to build in a metropolitan area, have to be constructed to the contours of the area in which they are located. Cellular systems are based on the use of a series of antennas that send straight line signals to cover particular neighborhoods.When a car passes from one area to the next, a different antenna picks up the signal. If the cellular system wins final FCC approval, AT&T plans to build the systems in 35 metropolitan areas serving 70 cities.

The AT&T system, called Advanced Mobile Telephone Service or AMPS, is a major experiment for the Bell System. AT&T's Chairman Charles Brown says AMPS is "ready to go and there's no doubt in my mind that it will go big -- once we have a green light from the FCC. AMPS will make mobile service widely available and offer quality comparable to regular telephone service."

The FCC still must resolve thorny issues on the mobile telephone question, including whether as many as half of these frequencies should be granted outright to the local telephone company. o

In the Washington area, there are no more than about 3,000 people that actually have direct-line telephones (or units that don't require connection first through an operator) in their cars. ARTS' Randolph, for one, thinks the Washington market -- full of people in high pressure, busy jobs -- is going to be enormous for the revolutionary mobile telephones.

"The demand for mobile service because of the very nature of the Washington community is as great as any place in the country," he said. o"There are a tremendous number of very mobile people in Washington and people are very communications conscious.

"Baltimore is just the reverse," he noted. "It is industrial and considerably more conservative. The initial demand in Baltimore will be less than in Washington."

The machinery for a cellular system, which is being built in this country by Motorola and several other firms, offers high quality sound in a convenient hand-held car set. Other than a slight echo, callers cannot tell the calls are being made from a car. In addition, the phone itself has a memory, allowing the caller to push a button and reconnect with a particular number, eliminating redialing.

While the FCC is nearing a decision on how to set up the cellular system, it also has made available dozens of new frequencies in the 800 megahertz band for more conventional radio-like telephones. A whole new category -- called specialized mobile radio (SMR) -- has been set up, offering a new flexibility and variety of services.

In Washington alone, there are now about 200 channels available under the SMR system, although many of them have been gathered up by communications firms and other entrepreneurs hoping to set up their own systems.

Although only 20 of these new channels are available per applicant, each channel could offer mobile phones to 2,000 people.

The availability of these new systems and frequencies could speed the development of newer types of communications products. For instance, International Business Machines Corp. is working on a tiny digital receiver that could use these new frequencies to transmit written messages to a pocket-sized instrument.

Just think what that could do to the message pad business.

If all these developments become reality, people who view a car as a sanctuary far from the ring of the office telephone may have to take up hiking. s