Low-cost carrier Southwest Airlines Co. LUV announced strong traffic numbers for Apr 2016, with revenue passenger miles (RPMs: a measure of air traffic) improving 4.7% on a year-over-year basis to 10.4 billion. Available seat miles (ASMs: a measure of capacity), on the other hand, grew 4.3% to 12.4 billion.Another important metric – load factor (% of seats filled by passengers) – improved 30 basis points (bps) to 83.6% during the month as traffic growth outpaced capacity expansion. Interestingly, this marks the highest ever load factor recorded by the Dallas-based airline company in the month of April.In the first four months of 2016, Southwest Airlines generated RPMs of 38.8 billion (up 8.4% year over year) and ASMs of 47.7 billion (up 7.9%), leading to a total load factor of 81.3% (up 40 bps).Southwest Airlines continues to expect operating revenue per ASM (RASM: a key measure of unit revenue) to increase modestly in the second quarter of 2016.The company reported encouraging first-quarter 2016 results, wherein both the top and bottom line surpassed the Zacks Consensus Estimate. Quarterly revenues were up 9.3% year over year to $4,826 million and above the Zacks Consensus Estimate of $4,818 million. On a year-over-year basis, Passenger and Other revenues increased 5.3% and 101%, respectively, while Freight revenues dropped 4.5%.Zacks Rank & Key PicksSouthwest Airlines currently carries a Zacks Rank #3 (Hold). Better-ranked stocks in the airline space include International Consolidated Airlines Group, S.A. ICAGY, Ryanair Holdings plc RYAAY and Air France-KLM SA AFLYY. All the three stocks carry a Zacks Rank #2 (Buy).Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report SOUTHWEST AIR (LUV): Free Stock Analysis Report RYANAIR HLDGS (RYAAY): Free Stock Analysis Report INTL CONS AIRLN (ICAGY): Free Stock Analysis Report AIR FRANCE-ADR (AFLYY): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research