The ornamental design for a displaydevice with a graphical user interface showing a vehiclenavigatortrafficlevelindicator element, as shown and described.
The sole FIGURE is a front view of a display device with a graphical user interface according to my design.
In the sole FIGURE, if and where shown, the broken lines illustrating the display device and portions of the graphical user interface form no part of the claimed design. Portions of the display device and user interface not shown in the sole FIGURE and not otherwise described in this specification do not form part of the claimed design.