Vehicle Turning Movements


The I-15 Franklin Street Interchange Project site handles various types of traffic. As the site exists within a designated truck route, turning movements must be taken into consideration during the geometrics design process. This is done by using AutoTURN, an addition to AutoCAD Civil 3D that allows the design team to overlay trucks and their respective turning movements onto geometric layouts to determine whether or not the design can accommodate such movements.


As stated above, the project study area is within a designated federal and state truck route. Due to this fact all truck turning movements must be taken into consideration for various reasons. The most prominent reason is these movements can have detrimental effects on the flow of traffic and facility elements such as curbs and gutters. For the purposes of this project, AutoTURN analysis was accomplished using a CALTRANS 2008 STAA-LONG truck template. The specific dimensions of this truck is illustrated in the figure below.

Vehicle Description


Standards exist as to the types of heavy vehicles encountered at the site. Caltrans follows the Surface Transportation Assistance Act (STAA) when designating heavy vehicle characteristics. With this guidance, the vehicle used in analyzing turning movements is a "STAA Long," a type of semitrailer truck that is 74 feet in overall length.

Roundabout Analysis

The Franklin Street Interchange Design Project has designed a total of four different interchanges. Three of these interchange designs, The Tight Diamond, Partial Clover, and the Single Point Urban Interchange (SPUI) incorporate a roundabout at the intersection of Canyon Estates and Elsinore Hills. The animated figures below show truck movements through the designed roundabout.

Roundabout Truck Movement

roundabout roundabout

Tight Diamond Interchange

The tight diamond interchange is one that is commonly seen. The figure below shows the most difficult turning movement for trucks on the designed interchange: a right turn onto one of the on ramps, specifically the southbound on ramp.

Tight Diamond Truck Movement


As depicted, the geometric design of this ramp easily accommodates the truck's turning movement. Similar analyses were performed on all other designed intersections for this alternative.

Partial Clover Interchange

The partial clover interchange is another design that is commonly seen. One of the most prominent features of this design are the loop ramps. While the loop ramps offer the ability to make the design more compact than a spread diamond, if designed improperly they pose a problem for truck movements through the ramp.

Partial Clover Truck Movement


As is shown in the figure above, the designed loop ramps easily accommodate truck movements.

Single Point Urban Interchange (SPUI)

The SPUI is an interchange that features one signal controlling for all turning movements through the interchange. Because of this feature, all turning movements onto ramps feature large radii, which allow for ease of truck movement.

SPUI Truck Movement


The figure above shows one such turning movement. The movement shown is the most difficult movement, as it is the one with which the truck's turning comes closest to failing the analysis.

Split Diamond Interchange

The split diamond interchange is the last design for this project. It is also the only design that does not incorporate a roundabout. Aside from the signalized intersection in place of the roundabout, all other intersections are the same. The designed ramps also meet Elsinore Hills at right angles as in all other designs.

Split Diamond Truck Movement


The movement shown in the figure above is a right turn from the southbound ramp that comes from the Main Street interchange to the north.


AutoTURN analysis is an iterative process. Geometric designs are created and then analyzed. If the design passes the analysis, then the design can move on to the next phase. However, if the analysis fails, then it must be altered to accommodate vehicle templates. While only the critical movements are shown, all movements were analyzed.