School of Civil Engineering

孙仁娟2012.4.17

URBAN ROAD DESIGN

Lecture 9- Design of Grade Separation-Ⅰ

• The High Five in Dallas, Texas, USA: an extreme example of interchange design. This is a complicated five-level stack interchange due to the close proximity of frontage roads.

Why should design interchange?

Objectives

• Identify and compare Grade Separation/interchange types, advantages/disadvantages;

• Learn Grade Separation design standards, specification, guidelines;

• Identify and describe Grade Separation/interchange design procedures and principles.

Terms

• Grade separation/Grade separated intersection is the process of aligning a junction of two or more

transport axes at different heights (grades) so that they will not disrupt the traffic flow on other transit routes when they across each other.

• Interchange: is a road junction that typically uses grade

separation, and one or more ramps, to permit traffic on at least one highway to pass through the junction without directly crossing any other traffic stream.

History of grade intersection

1928 first interchange

New Jersey State

Plan and design items

• Type• Volume • Turning movements• Capacity and LOS• Design speed: arterial , ramp, shift/distributed

lane • SSD• Speed changing lane length

• The goal of any well-designed interchange is to keep traffic flowing smoothly.

• To accomplish this, interchanges come in a variety of geometric forms with an array of design features.

• Understanding which form and features are best for a given application is the job of the transportation engineer.

Type of interchange

Full interchange Partial interchange

Four-way interchanges Three-way interchange

Four-way interchanges

• Cloverleaf interchange ( 苜蓿叶型、四页型、幸运草型） is typically a two-level, four-way interchange whereby all left turns are

handled by loop ramps (right turns if traveling on the left). To go left, vehicles first cross over or under the targeted route, then bear right onto a sharply curved ramp that loops roughly 270 degrees, merging onto the interchanging road from the right, and crossing the route just departed.

The major advantage of cloverleafs is that they require only one bridge, which makes such junctions inexpensive as long as land is plentiful. A major shortcoming of cloverleafs, however, is weaving, and the subsequent low-capacity of this design.

Cloverleafs also require considerable land consumption, hence they appear mostly in the United States, Canada, Germany, and the Netherlands. cellector and distributor roads are similar, but are usually separated from the main carriageway by a divider, such as a guard rail or barrier.

Cloverleafs are more often found along older highways, in rural areas, and within cities with low population densities.

A typical cloverleaf interchange in Ohio, United States.

Four-way interchanges• Stack interchange （环状型、或定向型） is a four-way interchange whereby left turns are handled by semi-

directional flyover/under ramps. To go left (right in countries with left-hand drive), vehicles first turn slightly right (on a right-turn off-ramp) to exit, then complete the turn via a ramp which crosses both highways, eventually merging with the right-turn on-ramp traffic from the opposite quadrant of the interchange. A stack interchange, then, has two pairs of left-turning ramps, of which can be stacked in various configurations above or below the two interchanging highways.

Stacks do not suffer from the problem of weaving but require massive construction work for their flyovers. A standard stack interchange includes roads on four levels. This is not only expensive but also creates an eyesore among local residents.

A multi-level stack interchange in Shanghai, China.

Four-way interchanges

• Cloverstack interchange （环状苜蓿叶型）

In the late 1960s, partial cloverleaf interchange (parclo) designs modified for freeway traffic emerged, eventually leading to the cloverstack interchange. Its ramps are longer to allow for higher ramp speeds, and loop ramp radii are made larger as well. For countries using right hand-drive, the large loop ramps eliminate the need for a fourth, and sometimes a third level in a typical stack interchange, as only two directions of travel use flyover/under ramps.

•

• Cloverstacks are cheaper to build than stack interchanges and are less of an eyesore for local residents. By using the loop ramps in opposite quadrants, weaving is also eliminated. However, cloverstacks require a lot of land to construct and the loop ramps are not as efficient as flyover/under ramps in terms of traffic flow. The cloverstack design is becoming more and more popular, and is commonly used to upgrade cloverleaf interchanges to increase their capacity and eliminate weaving.

Four-way interchanges

• Turbine interchange （涡轮型） Another alternative to the four-level stack interchange is

the turbine interchange (also known as a whirlpool). The turbine/whirlpool interchange requires fewer levels (usually two or three) while retaining semi-directional ramps throughout, and has its left-turning ramps sweep around the center of the interchange in a spiral pattern in right-hand driving.

Turbine interchanges offer slightly less vehicle capacity because the ramps typically turn more often and change height quicker. They also require more land to construct than the typical four-level stack interchange.

Four-way interchanges In areas with rolling or

mountainous terrain, turbine interchanges can take advantage of the natural topography of the land due to the constant change in the height of their ramps, and hence these are commonly used in these areas where conditions apply, reducing construction costs compared to turbine interchanges built on level ground.

Four-way interchanges

• Roundabout interchange （环岛型）• is a type of interchange between a controlled access highway such

as a motorway or freeway, and a minor road, in which the slip roads to and from the motorway carriageways converge at a single roundabout, which is grade-separated from the motorway lanes with bridges. A roundabout interchange is similar to a rotary interchange, which uses a rotary rather than a roundabout. Roundabouts may also be used in conjunction with other interchange types such as a standard or folded diamond interchange, though such use should not be confused with a roundabout interchange.

• Roundabout interchange

• Other/hybrid interchanges （混合型） • Hybrid interchanges use a mixture of interchange types

and are not uncommon. Their construction can consist of multiple interchange designs such as loop ramps, flyovers and roundabouts.

Three-way interchanges

• Trumpet interchanges （喇叭型）

have been used where one highway terminates at another highway. These involve at least one loop ramp connecting traffic either entering or leaving the terminating way with the far lanes of the continuous highway.

These interchanges are useful for highways as well as toll roads, as they concentrate all entering and exiting traffic into a single stretch of roadway. A double-trumpet interchange version can be found where a toll road meets another toll road or a free highway.

• Trumpet interchanges

A trumpet interchange on the Ottawa River Parkway.

• Directional T interchange （定向 T 型） uses flyover/under ramps in all directions at a three-way

interchange. Directional T interchanges are very efficient, but are

expensive to build compared to other three-way interchanges.

They also require three levels, which can be an eyesore for local residents. However, the T-interchange is preferred to a trumpet interchange because a trumpet requires a loop ramp which speeds can be reduced to as much as 25 MPH (US), but flyover ramps can handle much faster speeds.

• Directional T interchange

Semi-directional-T interchange in Lausanne, Switzerland.

• A semi-directional T does the same, but some of the splits and merges are switched to avoid ramps to and from the passing lane.

• The main curve (the curve immediately following the exit taper or preceding the entrance taper) should conform to the desirable ramp design speed, with maximum degrees of curvature.

• Superelevation:

The superelevation on ramps shall follow the same standards as those on the main roadway.

• Lane balance

The number of lanes beyond the merging of two traffic streams should not be less than the sum of all traffic lanes on the merging roadways minus one.

For entrance ramps bringing two lanes of traffic onto a road, the road beyond ramp entrance should be at least one lane wider than the road approaching the entrance.

History of grade intersection

我国修建立体交叉起步较晚，首先从解决城市道路交叉口的交通问题开始的。1955 年，武汉在滨江路上建成第一座部分苜蓿叶式立体交叉。1956 年，北京开始在京密引水工程滨河路上修建三座部分苜蓿叶式立体交叉。1964 年，广州市在大北路建成第一座双层环形立体交叉，该立交在 ,1986 年又改建为三层环形立体交叉。

Questions?