It usually takes way more than 2 years for traffic to increase 50%.  Colorado's population is increasing at a rate just under 2%/yr. That's about a 20 year cycle to see a 50% increase.

Now, is there usually a more than 2% surge on that specific road when that new 3rd lane opens? Yes, but not because there's a surge in overall traffic, but because traffic from alternate routes that were also congested shifts over to the newly-expanded route. People always forget about that part - the positive impact on the alternate routes. You can't look at it as a single road in a vacuum. It just doesn't work that way.

Again, purposely letting traffic just clog up worse and worse and refusing to add capacity is NOT a viable or responsible approach. That's like saying we should cap electricity usage in the metro are, and food usage, and water usage, to keep people from moving here. Yes, if we made life miserable here, people will stop coming. But of course that would be crazy.