A free-flow interchange is an interchange in which all roads are grade-separated, and where movement from one road to another does not require the driver to stop for traffic (for example, the interchange may not include traffic lights or unsignalized at-grade intersections).
Free-flow interchanges are less likely to induce traffic congestion than non-free-flow, but are typically more expensive both in money and in land.
Some free-flow interchanges bring additional problems such as weaving or passing lane/fast lane exits (i.e. left exits in areas with right-hand traffic, and vice versa) that may be necessary to avoid additional costs, but lead to congestion and accidents and ultimately to an upgrade to another type of interchange.
A free flow interchange design called double crossover merging interchange (DCMI) has received a patent.
It eliminates the disadvantages of weaving and of merging into the outside lane from which the standard DDI variation suffers.