A planned $1.7 billion expansion of I-405 should, in theory, give commuters almost a half-hour of their lives back by 2040. The project includes a new high-occupancy toll lane and a new regular lane in both directions from Costa Mesa to Long Beach. Groundbreaking is expected in 2018, and the lanes would open four years later. Traffic forecasts, experts say, are a mathematical exercise in probability, not certainty. “There are so many moving parts, it’s more or less an educated guess,” said Michael Zhang, professor of civil and environmental engineering with the UC Davis Institute of Transportation Studies. An overestimate of one factor, he explained, can create a cascade of inaccuracies.