940.12 Frontage and Backage Roads
Frontage and backage roads provide alternative access to property and help remove turning traffic from the through traffic on a mainline route. A frontage road provides alternative access at the front of properties while a backage road provides alternative access at the rear of properties.
Frontage and backage roads can dramatically improve safety and operations. However, a common mistake involves placing frontage or backage roads in close proximity to the mainline. Placing frontage roads very close to mainline roads can create additional opportunities for delay, congestion and crashes because insufficient storage (“throat length”) is provided for entering and exiting vehicles.
Space backage roads approximately 300 ft. or more from the mainline route. Measurements are to be taken from pavement edge to pavement edge. Backage roads are more advantageous than frontage roads because they minimize visual distractions and headlight glare on both the mainline and backage road. The backage configuration is particularly conducive to ownership by other governmental entities. Frontage roads may be more appropriate on freeways, especially those where MoDOT will maintain and operate the outer road system. See Clearance at Functional Area of Interchanges for further spacing recommendations.