Washington — Twenty-five cars traveling at top speeds approaching 180 mph will zoom around the National Mall this weekend when the racing series IndyCar takes over downtown D.C. for a race commemorating the nation's 250th birthday.The Freedom 250 Grand Prix of Washington, D.C., is expected to attract about 140,000 people on each day of the weekend. The event is the first major auto race in the capital in more than 24 years.The area around the track has been transformed over the past several weeks to make way for the construction of grandstands and enormous barriers, with large portions of the area blocked off to traffic and pedestrians. The race will disrupt several popular tourist destinations around the Mall and limit access to those with tickets to the event.President Trump is expected to wave the green flag to kick off the race, which has been in the works for more than a year. Here's what to know about the D.C. IndyCar race:What is IndyCar, and when is the race in D.C.?IndyCar is one of the world's leading open-wheel racing series, generally regarded as second only to Formula 1 in global popularity and prestige. The series is among the fastest racing categories, with cars capable of reaching 230 mph. Its premier event is the Indianapolis 500, one of three races that constitute the "triple crown of motorsport."This weekend's race will be IndyCar's first in the nation's capital and feature 25 drivers. The race itself begins at 1 p.m. ET on Sunday, but cars will be on track over the course of both Saturday and Sunday, given how an IndyCar race weekend is structured.The drivers will have two practice sessions on Saturday — the first at 9 a.m. and the second at 1 p.m. These sessions allow the drivers and their teams to get accustomed to the racetrack and try out different car setups and strategies, which is especially important on an unfamiliar track.