While fuel costs have driven up fares, other factors include airlines intentionally cutting back on capacity and flying fewer planes (which makes fewer seats available). Another factor is airline mergers, which have led to less competition on some routes, travel experts say. Overall across the country, air fares have gone up on average 6 percent to 10 percent, travel experts say. But to some destinations from Chicago they’ve spiked much more. Last year, among sample nonstop airfares for travel from Chicago in July to San Francisco, consumers could find fares of 8 round trip, but this year, that’s jumped to 0, according to BestFares Chief Executive Officer Tom Parsons. Nonstop flights to Denver have gone from 8 to 4, he added.