Dr. Fabio Sasahara
Navigating through a busy Two-Way Stop Control (TWSC) intersection can be challenging, especially on the minor street approaches subject to a STOP sign.
A median opening in a TWSC intersection, when present, can serve as a refuge for drivers desiring to cross the intersection from a minor street approach. It allows a two-stage crossing maneuver, greatly benefiting mobility and safety.
Two-stage crossing – HCM approach
Since the driver is only required to yield to the traffic in one direction at a time, the conflicting traffic computed for a minor street movement is significantly reduced, improving capacity and reducing delays.
Two user inputs are required to model a two-stage crossing in HCS:
- Major Street Median Type (undivided vs. divided), and which movements can be stored in a divided median (left only vs. left+thru)
- Number of vehicles that can be safely stored in the median
More details about these inputs are discussed next.
Major Street Median Type
Three options are available for the “Major Street Median Type” in HCS:
Major Street Median Type Options in HCS
An “Undivided” median type does not allow two-stage crossing maneuvers. Therefore, minor street vehicles must yield to conflicting traffic in all directions. This geometric configuration is expected to yield higher delays to minor street approaches.
For a “Divided” major street median, the user must also select which movements can take advantage of the median refuge.
If “Left+Thru” is selected, both left turns and through movements from the minor street will be analyzed as a two-stage crossing, as illustrated in the figure below:
A median opening serving both left and thru movements from the minor street
If “Left Only” is selected, the left turn movements will be analyzed as a two-stage crossing, but through movements will be analyzed as a one-stage crossing. This option applies to cases where the major street median is not wide enough to accommodate a vehicle stopped perpendicular to the major street. A typical example is a Two-Way Left Turn Lane (TWLTL) median, as shown in the figure below:
Left-turns using the TWLTL
Thru movements not possible in the TWLTL
Major Street Median Storage
If a “Divided” median is selected as previously discussed, the second required input is the number of vehicles that can be safely stored in the median opening. A higher value of median storage can increase the capacity of movements that perform a two-stage crossing:
Major Street Median Inputs in HCS
Engineering judgment is required to determine the inputs described in this article as a function of the local geometry of the subject intersection. Whenever possible, it is desirable to observe local field conditions for a proper evaluation of driver behavior using the major street median refuge.
