|This article does not cite any references or sources. (February 2014)|
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 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. Interchanges involving roundabouts are listed as non-free-flow because drivers entering a roundabout have to slow down considerably possibly creating backups.
In some cases, an additional requirement is imposed on interchanges to qualify as free-flow, especially on high-speed roads. This requirement is that the ramps be designed so that the driver does not have to slow down significantly in order to safely use them. Usually this means that 180- or 270-degree loop ramps are not allowed (since they usually require drivers to slow down to below 40 mph and sometimes even below 25 mph), which would disqualify the cloverleaf, cloverstack, and trumpet interchanges in favor of the stack, turbine, and (directional or semi-directional) T interchanges.
Examples of free-flow interchanges
- Stack interchange
- Cloverleaf interchange
- Trumpet interchange
- Cloverstack interchange
- Directional T
- Semi-directional T
- Turbine (whirlpool) interchange