The design and operation of the interconnected crossing should reflect the appropriate design vehicle. Typically, the longest vehicle permitted by the Highway Authority should be used as the design vehicle. Table 3-1 (reference from AASHTO’s Policy of Geometric Design of Highways and Streets, 2018) lists the AASHTO designation for potential design vehicle types on TxDOT Form 2304. The TxDOT Form 2304 automatically populates the design vehicle length and centerline turning radius based on the selected vehicle design. TxDOT Form 2304 also allows custom vehicle length to be added for special circumstances.

The Requestor should be aware of which vehicles are allowed at the crossing and associated intersection. Engineering judgement should be applied to determine the most appropriate design vehicle for a crossing as the design vehicle selection is a critical element and may affect certain preemption timing elements including the following:

Left Turns Towards the Tracks: Due to the difference in vehicle lengths, the time required for left turning vehicles to complete the turn will be affected.
Queue Clearance Time: The queue clearance interval must be long enough to allow the design vehicle to clear the MTCD. Due to the difference in vehicle lengths, the time required for the design vehicle to start up, move through, and clear the entire MTCD will be affected. This time is also affected by the time required to clear left turns