There are a number of options that allow you to run PC software on a Mac, some costing hundreds of dollars. Here are two options for installing Traffic Travis on your Mac that won't break the bank balance...
Virtualbox.org is one of the best virtual machines for Mac, and it's free. It is simple to install and has a reasonably small footprint (application size) but requires that you have a valid license for Windows if this is what you wish to install. You will also need the Windows installer disk to install from - there are sites that you can download a Windows disk image specifically for VirtualBox, but this isn't recommended.
Once you have Windows installed into your virtual machine, there is some configuration necessary which is well outlined in the help section on the website. After that, it runs exactly as a normal PC would - just inside a window on your Mac - or full screen if you wish.
You can then download Traffic Travis and install it as you would normally on a PC.
The second option if you do not own a license for Windows or have an installer disk, is to use an application from Code Weavers called Crossover available here.
For under US$40, Crossover is an emulator that runs as an application on your Mac, allowing you to install Windows exe files as if they were actually Mac applications. (They won't show on your dock though). There is also a time limited free trial which you can make good use of.
Once you have Crossover installed, the steps to installing Traffic Travis are quite simple:
Step 2: Click 'Install Windows Software' and select 'Other Application' from the first installation screen.
Step 3: Click "Choose Installer File" on the second screen and browse for the downloaded exe file for Traffic Travis.
Step 4: Create a new winxp "Bottle" (this is their term for the type of emulator you want to run under).
Step 4: Click the 'install' button and the Traffic Travis installer should run in just the same fashion as it would on a PC.
Step 5: Once installation is complete, tick the box asking if you wish to run Traffic Travis, and now you are up and running!