Lots of great tough climbs here in Colorado. The steepest I've done is Dory Hill out of Blackhawk - and YES it is legal and I've never been hassled. That's a solid three miles of 10% average. Then when you get back to Hwy 46 (GGCanyon) turn right and descend 3+ miles to Mountain Base. Left and you get 3 miles of climbing, of which over a mile averages 12% +. Parts of it show up as 18-19% on my garmin.