On real roads
A windmill interchange is a full interchange of two crossing motorways with eight ramps: a direct one for each right turn and a semi-direct one for each left turn, symmetrical about the crossing.1,2 German guidelines draw it with five bridges and no weaving sections.1
Germany's motorway design guidelines of 2008, the RAA, call it a Windmühle and recommend it where the turning traffic is heavy at all four corners of a motorway cross, side by side with the Maltese cross, a stack whose central structure has four levels.1 Poland's guidelines for road interchanges of 2022 base their table on the RAA, and recommend the windmill too where the heaviest traffic turns at all four corners.2
Compared with loop ramps, the Polish guidelines say, the windmill gives the left turns better conditions.2 Both guidelines count its bridges against it.1,2 In the German ones, its semi-direct ramps have small curves, so fairly steep grades and a poor view over their crests, and, like the stack, it gives no way to turn back.1
The German guidelines also draw modified windmills, with loop ramps for one or two of the left turns.1 Germany's federal motorway company plans to rebuild the Munich-East interchange, now a full cloverleaf, into a modified windmill with two semi-direct ramps for two heavy left turns.3
Where to see one
The Bonn/Siegburg interchange in Sankt Augustin from the air, 2010. Neuwieser, CC BY-SA 2.0, source, cropped, shown in greyscale Kreuz Bonn/Siegburg, A3 at A560, Sankt Augustin, DE. Map
Kreuz Mönchengladbach, A61 at A52, Mönchengladbach, DE. Map
The Roadform adaptation
In Roadform
Two highways cross; each left turn leaves with the right turn, loops on its own bridge over the highway it turns onto and joins it before the crossing.
Reading the connections
TF3 · M · On top: Crossing road · Traffic side: Right.
Scale and capacity
Size changes the road widths and the space the layout needs. The guide below uses the default arrangement. The layout reserves room for wider roads.
| Layout | Width × depth | Highest road | Lanes per direction |
|---|---|---|---|
| S | 863 × 863 m | 8 m | Main highway: 2 Crossing highway: 2 Ramps: 1 |
| M | 892 × 892 m | 8 m | Main highway: 3 Crossing highway: 2 Ramps: 2 |
| L | 892 × 892 m | 8 m | Main highway: 3 Crossing highway: 3 Ramps: 2 |
Which road takes the bridge?
Choose which road runs above the crossing. This choice works with every form and size.
TF3 · M · Traffic side: Right. Heights ×3.
Sources and credits
- Entwurf und Bau von Straßen – Teil: Straßenentwurf (lecture notes with the figures of the RAA 2008, the German guidelines for the design of motorways), Institut für Straßen- und Eisenbahnwesen, Karlsruhe Institute of Technology, 2013. web.archive.org
- WR-D-32-1 Wytyczne projektowania węzłów drogowych. Część 1: Wymagania podstawowe (Guidelines for the design of road interchanges, Part 1: Basic requirements), Ministry of Infrastructure, Poland, 2022. www.gov.pl
- Erweiterung der A 99 Ost: Bauabschnitt III, AS Kirchheim bis AS Haar (the Munich-East interchange), Die Autobahn GmbH des Bundes. a99-ost.de
Real-world photo credits accompany each image. Roadform figures and measurements use Transport Fever 3.