Travel conditions for the holidays hinge on a threatening strike at American Airlines that could seriously crowd railroads and other airlines. The weather is expected not to hinder road conditions in the Northeast.

"Everything will run as scheduled" to handle the Christmas exodus, said the Boston public relations office of American Airlines yesterday. The airline "anticipates nothing out of the ordinary" and is quite used to the threat of strikes, according to an official.

No extra cars will be added to the regular holiday runs of the New Haven Railroad even in the event of a strike or worse weather conditions. A representative admitted "the situation is terrible" and could become a "holocaust" if traffic is diverted from the airlines to the N.Y.N.H. & H. R.R. He claimed costs and equipment do not allow an increase in cars even if desperately needed.

The Automobile Club of Boston reported last night that virtually all main roads from Massachusetts to New Jersey and Pennsylvania are in good condition. The Massachusetts Turnpike is "clear and dry the entire length, with excellent driving conditions." The Merritt Parkway and Connecticut, New Jersey, and New York Turnpikes were said to be completely clear. The Pennsylvania Pike, according to the AAA, is clear except for icy spots at some entrances. Main roads Maine and New Hampshire are reported to have cleared driving lanes and snow-covered but sanded passing lanes. Vermont main routes are still slushy, but well sanded.

The only possible threats to automobile travel are a snow storm over the northwestern tip of New England and a larger Eastern-moving storm over Indiana today.