Where are you pulling this random 15% number from? And a ridiculous number too. It's more like 2% to 3% unless you spend your entire day traveling.

I just checked how much I pay for travel. My monthly travel expenses is £150 total. That's like between 2% to 3% of someone's net income (depends on how much net income you make).

Do you know it is impossible to 15% of your net income in travel because there is a weekly fare cap: https://tfl.gov.uk/fares/find-fares/capping

Worst case scenario, you are traveling too much every week to max out the fare cap across all travel zones. Your monthly total would be £244. That's like 3% to 6% of your net income. But this is the worst case scenario. If you are spending 6% of your net income on travel, maybe you should reconsider which zone you live in.

So seriously where are you pulling out this ridiculous 15% number from?