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 roundabouts). 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 interchange bring additional problems such as weaving or outside lane (passing lane) exits that may be necessary to avoid additional costs, but lead to congestion and accidents and ultimately to an upgrade to another type of interchange. Interchanges involving roundabouts are listed as non-free-flow because drivers entering a roundabout have to slow down considerably possibly creating backups.