Using Calendar's 'travel time' feature in Mavericks
One of OS X Mavericks' coolest features is the addition of a travel time calculator in Calendar. You may not know exactly how it works, or even know it exists, but rest assured: It's there waiting for you.
Simply open the calendar application and open an event. Regardless of whether it's a pre-existing event or not, simply look for the "Add Location" info box below the event title. When you click this box, your Mac will tell you the computer would like to access your current location. This allows the travel time calculator to know how far you are from your location. You'll have to weigh any privacy concerns you might have against your desire to use this feature.
Once you've allowed your location to be known, type in the address of your meeting/concert/BBQ. You'll see Calendar make suggestions. Even if you type in the address perfectly, you have to select one of Apple's suggestions for this feature to work currently.
In the event box, click on the date of your event. It'll look like this.
Once you click that button, a new list of options will appear, allowing you to customize when the event starts and ends, if it repeats and, of course, travel time. Click on travel time and, based on your distance from the location, Calendar will let you know how long your route will take to walk or drive.
Now, knowing how long it will take you to get to your impending destination, Calendar can alert you not only 30 minutes before you need to leave, but also give you a reminder exactly when you need to leave. Simply set your alert to the handily titled "When I need to leave" option.
Our friends MacObserver have written a handy tutorial that will answer any of your more complicated questions about the process. You can find that right over here.
