Vehicle Turning Movements


A variety of vehicles traverse the project site, including heavy trucks and recreational vehicles. Large vehicles take more time and space to maneuver through the intersections of the proposed alternatives. Therefore, there is a need to evaluate horizontal alignments for heavy vehicles. This is accomplished using Transoft Solutions' AutoTURN, a program that overlays the footprint and sweep path of a variety of vehicles onto the proposed designs.


The project study area is within a designated federal and state truck route; Interstate 15 serves as a major logistics travel way for cargo goods. Heavy vehicles are expected to navigate the area. Ingress and egress of such vehicles at Railroad Canyon Road must be accommodated so as to not create traffic delay or damage roadway facilities. All proposed geometric horizontal alignments for each alternative are to be analyzed for heavy vehicle accommodation. If the design fails to accommodate the vehicle, then recommendations are made to modify the geometric designs. This is an iterative process continuing until the design meets the vehicle requirements.

Vehicle Description


There are standards 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 "STAA Long," a type of semitrailer truck that is 74 feet in overall length.

Continuous Flow Interchange

The CFI's unique feature is the crossover movement onto the freeway ramps. The movement is a protected left turn followed by a yielded left at the freeway ramp. AutoTURN analysis shows that difficulties were made when a truck initially makes the crossover movement. Drivers had to make a sharp left followed by a sharp right so as to clear their trailer when entering the opposing traffic lane. This was mitigated with a barrier that separates the crossover lane from opposing traffic.

CFI Truck Movement 1

The yield movement onto the freeway also generated issues for heavy trucks; the turn is tight, creating risks where the truck may overrun its path or come into contact with sidewalks. These issues were mitigated by widening the entry point where trucks would begin their turn.

CFI Truck Movement 2

Diverging Diamond Interchange

AutoTURN analysis shows that the DDI's braiding movement for through traffic accommodates heavy trucks sufficiently. Problems arose for trucks exiting the freeway and making a left onto the facility. The turn is sharp as of the result of moving the street inwards. A truck's sweep path encroaches onto other turn lanes, hindering other vehicles on adjacent lanes from performing the same turn. This was mitigated by widening the turning radius guidelines, thus creating more space for a truck to perform its movement and not conflict with other vehicles.

DDI Truck Movement

Single Point Urban Interchange

AutoTURN analysis shows that the SPUI best accommodates heavy vehicles due to the ramps having large radii up to the single point of intersection. Trucks maintained their lanes and faced no troublesome maneuvers. No adjustments to the geometric horizontal alignment were needed.

SPUI Truck Movement