An already sluggish year at the box office keeps getting worse. The $20 million opening for Ridley Scott’s epic “Kingdom of Heaven,” while within 20th Century Fox’s dampened domestic expectations, led a weekend in which theater traffic slowed to a crawl. In recent years the first weekend of May has seen a big expansion in the marketplace. But if Nielsen EDI’s estimate of $83 million holds when final figures are tallied, it would be the worst weekend of an already listless year. It is also 26% behind last year’s summer kickoff frame, when “Van Helsing” opened to $51.7 million –perceived as a disappointment at the time. Warner Bros.’ slasher remake “House of Wax” opened to $12.2 million from 3,111 locations, a healthy number for the $35 million film but on the low end of industry expectations heading into the weekend. Opening a bit stronger than expected was Lions Gate’s “Crash,” which collected $9.1 million from 1,864 venues. Paul Haggis debut helming effort tied for third place with last week’s champ, Disney’s “The Hitchhiker’s Guide to the Galaxy,” which dropped 57% in its second week. Universal’s “The Interpreter” finished out the top five with $7.5 million in its third week. The Nicole Kidman-Sean Penn thriller was down 46%; its cumulative take is $54.1 million.