I haven't used the Apple's traffic widget because my iPad mini2 stays at home most of the time and my iphone 4S is still running an old iOS 7.1.2 version.
I have read that there is still a bug with if i read this topic.
There is also an alternative if you use the car which is called 'ETA'.
For other public transports app check the sticky on widgets at 'Travel' to see a few options.
The Apple traffic widget is simply a line of text that gives you an estimated time of travel to locations that you typically frequent on a regular basis. It shows up ad hoc.
Make sure that it's enabled by going to Settings>Privacy>Location Services>System Services
A widget can just give you a quick glance view of a route or a few routes whenever you just pull down the notification center, which can even be done with the phone locked--so makes it for a much simpler and faster check of traffic essentially.
I get specific traffic info from Apple when I put the address in my calendar entry. When I make an appointment on my calendar the second line of text is location. If you use the correct address of your meeting location the notification screen will tell you something like "looks like normal traffic on your drive to....." We don't have many issues here with traffic so it almost always says normal traffic but in addition it will tell you what time you would need to leave to get there on time. Handy.
Hopefully, google now gets a widget soon. The apple traffic details are horribly inaccurate around DC. It always says the same 37 minutes to work, regardless if the highway is on fire and backed up for miles.
There's an app called ETA which lets you save locations, and can show you roughly how much time is needed to get there. Sort of like a bookmark for maps, as you can click on those locations in the widgets.