For rural areas, 42-46 feet at minimum, preferably 60-70 feet or wider.
Anything ~50 feet or smaller should utilize at least a cable guardrail in the median to prevent median crossovers.
Here's an
example of a properly done 46 feet median on I-40 between Raleigh and Wilmington built throughout the 1980s and early 1990s with a continuous cable guardrail in the median. The posted speed limit is 70 mph. This is the standard design for rural freeway construction in North Carolina, though occasionally, such as on the under construction NC-540 project and the upcoming I-73 Rockingham Bypass, a wider 70 feet median is utilized. Even with a wider median of 70 feet, North Carolina will still utilize
cable guardrail in the median as an added precaution.
Texas has been taking a different approach to medians on new construction largely and instead utilizing 10 foot left paved shoulders and a jersey barrier dividing the lanes of traffic. Newer interstate highways such as I-69 within the state are being constructed like this, and future upgrade projects calls for maintaining this typical section for at least 40+ miles in areas. Here's an
example on a recently completed segment of I-69 outside Bishop (will not be signed until the Driscoll bypass is completed in 2022 linking to I-37). Not my ideal preference, but I feel like as long as a full left shoulder is provided to add "breathing" room, it doesn't pose any issues. I had more issues with the design until I actually drove on it, and it's actually not that bad aesthetics and design wise. In most cases, there are, but in some interchange projects (converting a single intersection into a grade-separated interchange), they've only provided a 4 foot left shoulder.