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A stack interchange is a free-flowing junction between two or more roads that allows turning in all directions. This is the best type of four-way interchange for the driver and in terms of capacity, but is also the most expensive, both in terms of land and in terms of construction.
Four-level stack
The four-level stack (or simply four-stack) has one major road crossing another on a bridge, with connector roads crossing on two further levels. This type of interchange does not usually permit U-turns.
The first stack interchange in the world was the aptly-named Four Level Interchange, built in Los Angeles, California at the junction of U.S. Route 101 and California State Route 110. Another American example (although slightly augmented because of ramps to and from side roads) is the Tom Moreland Interchange in DeKalb County, Georgia.
Though there were plans for more such interchanges, the Canadian province of Ontario has only one four-level stack interchange--the interchange between Highway 400 and Highway 407.
In Great Britain there are three four-level stacks: at the junction of the M4 and M25 near Heathrow Airport in London, at the junction of the M23 and M25 to the south of London, and at the junction of the M4 and M5 near Bristol.
Cloverstack
An alternative, the three level "Cloverstack" is commonly used to redesign cloverleafs to reduce weaving. This happens when two loop ramps from a cloverleaf interchange are removed and replaced with two flyover ramps. They are most common between two freeways or expressways when there are great traffic disparities between traffic (the movements with lower traffic get the loop ramp).
A good example of a two-loop cloverstack is at Highway 407 and Highway 410 in Toronto, Ontario, Canada *. Originally, a four level stack were planned for this junction but it was reduced to a cloverstack to save costs.
A good example of a one-loop cloverstack is at SR 528 and SR 417 in Orlando, Florida, United States. *
Some cloverstack variants may still include weaving and be four level. This happens when two loop ramps are removed from the same "half" of the interchange. However, even in such a situation the ramps can still be designed to prevent this. This is shown in the interchange between Interstate 64 and Mercury Boulevard (U.S. 258)--originally a cloverleaf and being converted into a cloverstack (set to be complete in summer of 2006).
"Stack interchange"