Why Not Build a Cloverleaf?
There are many technical elements for a transportation designer to consider when designing an interchange. Some key factors that often negate the use of a full cloverleaf include:
- Weaving and traffic volumes
- Safety
- Land availability
In urban areas, left turning volumes are often very high at major intersections. On a full cloverleaf design, all left turning movements use the loop ramps and generate weaving maneuvers. The problem arises in the area of the weave between two loop ramps. If the volume entering the loop is high, coupled with a high volume of exiting traffic from an adjacent loop, then the weave required to get from one loop to the next cannot be made in a smooth transition. Eventually, vehicles begin stopping in the weave area which contributes to serious accidents, particularly on major high-speed thoroughfares and when there is only a short distance available for weaving.
As a result, The City of Calgary has removed loops in two cloverleaf interchanges at Blackfoot Trail/Glenmore Trail and at Sarcee Trail/Trans Canada Highway. Similarly, The City of Toronto has removed all of its cloverleaf interchanges.
Another factor influencing the choice of interchange design is the amount of land a full cloverleaf requires. To build a cloverleaf safely and in accordance with accepted design standards, it requires a large area (please refer to the diagram available for download on the right of your screen).
The use of a full cloverleaf for interchanges on major roadways is not recommended for many city situations due to high traffic volumes. Cloverleafs are most appropriate for applications in rural areas with low turning movements and low traffic volumes.
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