Do highway expansions reduce traffic congestion?
Not in the long term, but it may provide some congestion relief in the short term.
In Europe, Japan and the US, highway expansions have been found to have essentially no impact on traffic congestion in the long term (defined as 15 or 20 years). These studies find that traffic increases roughly proportionally with highway capacity. In other words, doubling the length or width of highways causes traffic to also double, providing no congestion relief.
This finding is due to 'induced demand' as additional road capacity induces more driving by residents, commercial activities, and migration to cities with better road infrastructure. In Europe, road pricing and public transportation in the form of railroads has been found to reduce these induced demand effects, while in the US, busses have been found to have no impact.
Most studies have focused on the long-term effects of highway construction. Recent (unpublished) research suggests that highway widenings in the Netherlands provide some congestion relief in the short term (within six years).