Horizon's Bike Trail is indeed open to the public. It is a 5.8 mile course that sits atop a bluff overlooking the Arkansas River. Expect about 95% singletrack, fast dirt roads, hair-raising ascents, fast downhills, and some rocks to navigate. Ride along a ridge overlooking the Arkansas River. Not recommended for beginers.
How to get there:
Horizon Center is tucked away in the beautiful hills of South Central Kansas. Follow Highway 77 into Arkansas City. Near the south end of town is Madison Street. Turn east onto Madison and follow that road (which changes to County Road 12 at the city limits) for approximately 3.5 miles. At that point there will be a fork in the road where County 12 will curve to the left. You will need to curve to the right onto County 6. Follow County 6 for 2.5 miles. Turn right at the large Horizon sign and go 1 mile south to the Horizon entrance.