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.
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.
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
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
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
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
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
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.