About 40% of flights were canceled Friday at all Paris airports, and tens of thousands of passengers were rearranging plans at the height of the summer travel season because of a strike by French air traffic controllers seeking better working conditions.

Traffic jams are forecast on key French routes this weekend, as well as busy railway stations and issues at the airports as the after-effects of the this week’s two-day strike are still being felt.

The timing of the strike is especially acute, with Friday being the last day of school in France before the summer vacation and many families contemplating an early departure