Turbine interchange

A turbine interchange, also called a whirlpool, joins two freeways with a directional ramp for every turn.^1,2 TxDOT describes its left turns as right-exit ramps that sweep round the centre of the interchange in a spiral.¹ A four-level stack takes both freeways and all the left turns through one point in the centre; the turbine needs fewer levels, usually two or three.^1,3

In the United States the turbine was rare in 2013. That year FHWA called the one being built where I-85 meets I-485, in the Charlotte area, the first in North Carolina and one of only a handful in the country.⁴ In a 2014 presentation, NCDOT and Lane Construction pointed to similar turbines in Tampa, Florida, and at Thorpe in the United Kingdom.⁵

For Charlotte, NCDOT's slides list a four-level stack's challenges: high bridges over heavy traffic, tall retaining walls and two million cubic yards (about 1.5 million m³) of earth to bring in.⁵ The Lane/STV team proposed a two-level turbine, with smaller bridges, flatter road profiles and much less earthwork.⁵ In 2013 FHWA gave it 18 bridges, three times more than a previously considered design; their smaller sizes were expected to make it easier to build and maintain, saving an estimated $30 million by NCDOT's figure.⁴ NCDOT's 2014 slides count 19 new bridges.⁵

Two state DOTs have chosen turbine ramps to rebuild a cloverleaf. TxDOT is turning the cloverleaf of US 69 and SH 73 near Port Arthur into a turbine, entirely within the existing right of way.⁶ At I-16 and I-95 in Georgia, GDOT found by benefit/cost analysis that a full rebuild was not fiscally responsible, and chose to replace two of the four loops with turbine ramps, a partial turbine and the first in the state.² NCDOT's guidance sums the form up: many bridges, but a smaller footprint than some other designs, bridges that are not tall, ramps that can be faster, no weaving and no exits or entrances on the left. It should be considered when an interchange is built or rebuilt where two freeways cross.⁷