Four-level stack

A four-level stack joins two freeways with a direct ramp for every turn, and no loops.^1,2 The two freeways cross at the centre, and the left-turn ramps pass the same point on two more levels, so four levels of roadway pass one another at one point.^2,3 In Los Angeles they did so in a single bridge structure.³ It is also called a stack or a four-level interchange, and is one kind of fully directional interchange.^1,2,4

It began in Los Angeles, where the Hollywood Parkway was to cross the Arroyo Seco-Harbor Parkway at the edge of the Civic Center.³ In 1944 engineers of the California Division of Highways described the four-level grade separation they had developed for that site as an entirely new type of interchange; it grew from an idea first suggested by W. H. Irish, the district location engineer.³ In place of the cloverleaf's 270-degree loop to the right, it gave every movement a simple turn, and the Division's magazine expected it to have more capacity than a cloverleaf.^3,5 The structure was completed in 1949, the first of its kind.^5,6 On 22 September 1953 all its levels and connecting roads were put into operation for the first time.⁷

It was offered as a design for other sites too: compact, efficient and safe, and at comparable cost where conditions justified it.³ Early in 1953 it was, so far as was known, the only four-level structure built.⁸ In the 1940s and into the mid-1950s, Los Angeles began its freeway system with all-directional interchanges between freeways, and Leisch and Morrall write that the form, first developed there, is often called the California 4-Level Stack.² TxDOT's manual gives it as the four-level directional interchange; carrying the frontage roads through it as well makes a five-level one.¹ An FHWA report of 2018 says the northeast junction of I-85 and I-285 in Atlanta typifies four-level interchange design.⁹

TxDOT's manual says fully directional interchanges are usually justified where two freeways cross.¹ Directional interchanges in general are most often built in cities and suburbs; in the country there is generally too little traffic to justify direct or semi-direct ramps in every quadrant.¹⁰ The stack's ramps keep close to the way each driver is going: trips are shorter, speeds and capacity higher, weaving is reduced or removed, and there is no loop in which drivers lose their sense of direction.^1,11,12 On land the manuals differ: INDOT says directional or semi-directional interchanges may need less right of way than a cloverleaf; MoDOT, that directional interchanges need a large area, though comparatively little more than a cloverleaf.^12,13 It gives the highest capacity possible, but its levels of structure make it extremely costly to build, and California's manual asks for its benefits to be fully substantiated.^4,11,12 In 1950 the Los Angeles interchange, with its land and the twelve 'satellite' bridges it needed nearby, represented an investment of $5,000,000; Cressy, an assistant district engineer, wrote that an interchange so costly was justified only where large volumes of traffic must be given an easy interchange between freeways.¹⁴

In Germany, a 1977 study compared cloverleafs with interchanges with direct ramps, which it called Maltese crosses (Malteserkreuze). These cost about 50 % more to build but gave only a little more capacity overall, as their right turns fared worse than their left.¹⁵ Small design improvements to cloverleafs, for only about 5 % more cost, could raise their capacity by up to about 50 %, and the study judged that cloverleafs would as a rule still be enough to join autobahns.¹⁵