Southwest ( LUV) will begin flights from New York's LaGuardia Airport to Baltimore and Chicago on June 28, entering the world's biggest travel market 38 years after it began flying. Southwest will not formally fly shuttle routes, but it will fly three times daily between LaGuardia and Baltimore, the third Washington area airport, as well as five times daily between LaGuardia and Midway. Fares will begin as low as $49 for the Baltimore flights and $89 for the Chicago flights with a 14-day advance purchase. "Customers in both Chicago and the DC area now have a new low fare choice when planning trips into LaGuardia," said CEO Gary Kelly, in a prepared statement. Through the two cities, Southwest will offer connecting service to more than 45 destinations, including Las Vegas, Denver, Los Angeles, Seattle and San Diego. Southwest recently acquired takeoff and landing slots at LaGuardia, paying $7.5 million to the estate of its defunct partner ATA Airlines. Its arrival at LaGuardia could force Delta ( DAL) and US Airways ( LCC) to offer competitive fares on their Washington shuttles, although both offer more frequencies and serve more convenient Washington Reagan National Airport. Once focused on seeking out underserved airports, Southwest is increasingly operating to congested airports and other carrier's hubs, threatening its ontime performance and increasing costs as it operates in delay environments. However, revenue opportunities are highest at the airports that serve the most passengers.