Open Photoshop and create a new image.

Select the Polygon tool

Set it to 3 sides.

Draw a triangle then rotate it so its in the right position.

Now to make the corners rounded.

A simple way is to Ctrl + Click the layer then click Select » Modify » Smooth and set to 15px.

But you get a bad result on 2 of the corners:

To make the corners come out properly Ctrl + Click the layer then select the channels panel.

Then click the "Save selection as channel" button:

Left click on alpha 1 to select it. Press Ctrl + D to deselect the selection.

Then go to Filter » Blur » Gaussian Blur and set to 15px.

Then go to Image » Adjustments then levels.

Use these levels.

Ctrl + Click the Alpha 1 layer to select the rounded triangle.

Change back to the layers panel and create a new layer.

With the selection still active fill it with black.

With the selection still active fill it with black.

Delete the original triangle.

Right click the layer and select blending options.

You should now have a nice smooth rounded sign.

Add an image or text to your sign and you are complete.