The cloverleaf is one of the oldest types of higher-capacity interchanges in the US. The basic idea is that each of the twelve possible movements (right, left, and straight from each of the incoming roads) can be made in the interchange without stopping for cross traffic. They can be found just about anywhere in the United States; almost every state has at least one, although some states (Texas and California, particularly) tend not to use them to any great extent. The first one was built in Avenel, New Jersey in 1929. At the time, it was the intersection of State Routes 4 and 25; it's now the junction of US 1 and 9 and NJ 35. This is the interchange is shown above.
In the early days of freeway and interstate construction in the United States, cloverleaf interchanges were regarded as a great way to move traffic between two freeways or between a freeway and a busy crossroad. Traffic could keep moving without stopping through the entire interchange, regardless of its destination. Cloverleaf interchanges were comparatively cheap - the only other free-flow intersection with less bridges involved is a large traffic circle, which, (luckily) early engineers realized, would be a huge disaster if placed at the intersection of two high-speed freeways. A little bit of land was needed for the 270-degree loop ramps, but this was no problem; the loops could just be made smaller if needed. The interchange wouldn't make a huge impact on the skyline (and the project budget) like a four-level stack might. As a result of all of these advantages, cloverleaf interchanges continued to be built all over the country at many freeway intersections and where a diamond interchange didn't have enough traffic capacity.
However, there are a few problems with cloverleafs. (These are the reasons that they're not present at every single freeway-to-freeway intersection across the country.) First, the loop ramps must go around 270 degrees of rotation. To build the loop ramps, a trade-off must be made for size vs. utility. A ramp with a much larger radius can be taken at higher speeds and thus move more traffic, but it takes up a lot more real estate. A smaller loop takes up less space at the cost of slower speeds; taken to an extreme, small loop ramps can result in a danger of tipping for truck traffic.
The second (and, in my opinion, most major) problem with cloverleaf interchanges is the weaving that must occur between the left-turn movements. The basic problem is that two types of traffic is switching lanes in the middle of the interchange: traffic that has just entered from the right from the first loop ramp, and traffic that is about to exit to use the second ramp. Both of these streams of traffic must switch places between the right lane of the through highway and the auxiliary entrance/exit lane. In addition, the entering traffic is trying to accelerate to merge into the through traffic, while the exiting traffic must slow down to negotiate the tight turn on the loop ramp. These two factors are the cause of many traffic backups and an increased risk of collisions.
There are some solutions that retain the look of a cloverleaf interchange, but fix many of its problems. They are shown below.
|The "Parclo" Interchange: "Parclo" is short for "partial cloverleaf;" that's exactly what this interchange is. It's half cloverleaf, half diamond. It only works at the intersection of a freeway and a local road (or, I guess, a local road and another local road.) Basically, the weaving on both highways is solved by eliminating two of the cloverleaf ramps, and replacing them with signalized intersections on the local roadway. (This wouldn't work at an intersection of two freeways, due to the fact that some through traffic must stop.) It's also possible to just eliminate one loop ramp and retain some of the weaving, or to eliminate three loop ramps and be left with more signalized intersections. Here, there are one offramp and two onramps in each direction; it's possible to mirror the interchange such that it has one onramp and two offramps instead. This orientation is usually preferred, however, because it allows all exiting traffic to leave from a single point, and has no traffic waiting in the center of the more minor roadway to make the left turn onto the onramp.|
The interchange to the left is at the intersection of ON 407 and Britannia Road East, west of Toronto, Ontario. (Thanks to Google Maps for the image.) Parclos are very popular in Ontario, to the point that the entire length of ON 407, the new Express Toll Road, uses them almost exclusively at its intersections with local roads. They also abound on ON 401 north of Toronto.
|Collector/Distributor Roads: Collector/distributor roads, or C/D roads, are a way of keeping the same structure of a cloverleaf interchange but removing weaving conflicts from the main through highways. A ramp splits off before the interchange, giving traffic access to both loop ramps and both right-turn ramps. These lanes are generally seperated from the mainline, usually with a grassy median or at least a concrete barrier. The weaving occurs on this C/D road instead of on the highway mainline; the only time turning traffic has to interact with mainline traffic is at the beginning and end of each C/D road. The view on the left shows an interchange with three collector/distributor roads: one on each direction of I-40, and one on US 1 southbound.
To the left is an aerial view (from terraserver-usa.com) of the intersection of I-40 (NW-SE) and US 1/US 64 (SW-NE) southwest of Raleigh, NC.
|Semi-Directional Ramps: Also known as flyovers, these ramps provide an alternate route for one of the left turn movements. Instead of a loop ramp, these ramps split off along with a right-turn ramp. They then turn to the left through three quandrants of the interchange and join the other highway via a merge into another right-turn ramp. They tend to favor a certain direction of travel, allowing for a higher-speed connection on one left-turn movement; they eliminate weaving on two of the four through roadways, namely the two directions which it connects. |
To the left is a picture (from terraserver-usa.com) of I-787 (to the south and east), NY 7 (to the west and east), and NY 787 (to the north), in Green Island, north of Albany, NY.
|Cloverstack: This is essentially a cloverleaf interchange with two semi-directional ramps, resulting in an absolute elimination of weaving. It can also be described as a four-level stack with one level taken off and replaced with a pair of loop ramps. Being a hybrid of the two interchange designs, the name "cloverstack" was given to it. (According to kurumi.com, the name was coined by fellow roadgeek Nathan Perry, who incidentally runs another great interchange site at his website.) It is less obtrusive and cheaper than a full four-level stack, while being much more functional than a cloverleaf.
To the left is a picture (from terraserver-usa.com) of I-55 (north-south), I-255 (east), and I-270 (west), which is in Green Park, south of St. Louis, MO. This is one of the only full cloverstacks in the US, although these also are common on the new ON 407 north of Toronto, Ontario.
|"Braiding" loop ramps: This is a somewhat bizarre and very rare treatment for a cloverleaf interchange. It is a result of California's decision that all cloverleafs, when they are rebuilt, should be converted to parclos or otherwise modified to eliminate weaving. Here, weaving still exists on the cross-road, which isn't usually the problem anyway. To eliminate weaves on the freeway, though, the left-turn exits leae the mainline well in advance of the interchange. Each one then drops below grade and passes under the incoming loop ramp, then rises to pass over the cross street, and then finally makes a 270-degree turn down to the local street. It's not a bad idea, really - I'm almost surprised I've never seen it anywhere else. (If there is one somewhere, let me know so I can remove my foot from my mouth and tell the truth.) |
California's always one to try new things. The interchange to the left is at the intersection of CA 170 and Victory Boulevard in the northeastern suburbs of Los Angeles. It's the only full cloverleaf I know of that uses this method of reducing weaving. The next interchange to the north, at CA 170 and Sherman Way, is similar to this one; the northbound (eastern) half has the braided loop ramps, but the southbound (western) half of the interchange has only two ramps, creating a half-diamond with a stoplight at the intersection with Sherman Way.