Directional T interchange

A directional T is a three-leg interchange, the kind usually built where one highway begins or ends at another.¹ None of its ramps is a loop: the right turns take direct ramps, and each left turn a semi-direct one, which normally exits to the right first and reverses direction before it joins the other highway.²

When freeways of real length were built in the late 1930s and early 1940s, the cloverleaf, the diamond and the trumpet were the main interchange forms.³ By the time the Interstate system began, in 1956, interchanges were sorted into a few basic types, and the three-leg ones were the trumpet, the directional T and the Y.^3,4

In the 1950s some system interchanges got exits and entrances on the left as well as the right.³ Experience and research showed that left exits have more crashes than right ones, and by the 1990s exits and entrances all on the right were generally accepted practice.^3,4 On a directional T the left turns normally exit on the right as well.²

The Massachusetts design guide finds the directional T best suited where both left turns are fairly heavy; where every turn is heavy, it points to a fully directional form.¹ A semi-direct ramp carries about 1,200 to 1,500 vehicles an hour, where a loop is limited to 800 to 1,200.² The same guide counts the directional T among the costliest three-leg types, as its centre needs several structures for three levels of traffic, and like every three-leg interchange it is very hard to expand to four legs later.¹