1. Open new image Width: 200, Height: 40.
2. Select the color values for foreground and background:
3. Zoom in and use Rectangular Marquee Tool (M) to create selection and fill it with yellow like here:
Keep in mind that you will need two selections of that size in the image so make sure you have plenty of space.
4. Do not deselect yet. If you did then make step backward in history. Create new layer and fill the selection with black color. Now use Move Tool (V) to move black layer down. Make sure that there is no empty space left between two layers. You should have the following:
5. Merge two layers in one or press Shift+Ctrl+E (Merge Visible). Go to Edit -> Free Transform. Rotate the layer about 75 degrees.
6. Now you got one layer ready to use. Duplicate it and then use Move Tool (V) and move duplicated layer to the right. IMPORTANT!!!: Make sure there are no empty spaces (gaps) between two layers. Otherwise, it will cause ugly holes and edges when this pattern will be used to fill various areas. Also, make sure that layers DO NOT OVERLAP each other as that will cause uneven edges when using this pattern to fill.
So now we have two layers placed to each other without gaps or overlaps. Duplicate our very first layer again and place it next to the right of first duplicated layer. In the end you should end up with this:
7. Tricky part. We need to select an area of image that is being repeated over and over thus we can get the pattern and use it to fill large areas later. The easiest way to find that area is to visually select easy to notice object and see where it is being repeated again. Look at image below, the visual object (dots) I found are being pointed at.
Now simply select repeating area. Then go to Edit -> Define Pattern and give it a name.
8. Here's the actual pattern image:
9. To test your new pattern create new image with the same height(!!) and with any width. I used 500x40.
Or try adding styles and various effects
10. If you want your caution bar to be vertical simply rotate canvas 90 degree or whatever you need.