Depending on where you are you may want to carry more than $40. Last week was about the fifth time my car was parked in a large city's downtown garage where their credit card processing systems were down. Because I had cash on me I was able to pay and be on my way. I saw many angry parkers sent off to ATMs.

My baseline cash for a trip is based on (one expensive large city downtown parking lot) + (gas to get home) + (a sandwich or two). I usually bring a lot of cash home for next time.