How often you walk your cat is important. With each walk your cat stores additional information about the world and how it works. Depending on your cat, it may take dozens of times to learn that a certain combination of stimuli is safe and even longer to habituate on it so that it doesn't disturb its walk.
Don't give up.
If it's sounds -- traffic being a major one -- you can help your cat out by recording the sounds in the highest fidelity you can obtain, and playing it for your cat during play time. Turn on the sounds and then bring out its favorite interactive toy and encourage play. Create a 15-minute loop of the sound and end your play period when the sound ends. In no time, the sounds your cat dreaded will signal the beginning of something good and your cat may run to the sounds rather than away. If your cat is more responsive to treats than play, you can substitute it's favorite treat for play. Treat your cat at the beginning of the sound loop, around the middle, and the end. Don't over treat, just try to make sure your cat associates the treat with the sound.
So back to frequency. Yeah, it seems like an important consideration for most cats. If you can, walk your cat every day. I know, it sounds like a lot, but dog walkers manage it, so you can too. If you only take your cat once a week, or less, the it will likely increase the time it takes for your cat to habituate on the scary stuff. Pick a time of day when you can generally take you cat for a walk and stick with it if it's a problem for you. That said, on the weekend or your days off, try to pick a different time of day. Your cat will respond differently at different times of the day. For instance, being crepuscular, cats are most eager to hunt at dusk and dawn. Your cat will likely be more alert and a little less nervous at those times. If you have a very skittish cat, these may be the best times of day to train your cat. Keep a journal in the early days, being careful to note time and location so that, over time, you see a pattern forming. In the early days it's okay to focus on the times/places your cat seems most comfortable with, just don't focus on them exclusively. You'll need to challenge your cat regularly; even if just for a minute or two.