Switchback Design

[ataman]

Hi

Are there any rules of thumb for the design of switchbacks (some may know it as hairpin bends) up steep slopes.

I have a road which has a gradient of 17% and need to design a switchback. What would be the recommended minimum/maximum for the following

1. Radius
2. Road cross section gradient going around the corner
3. Road longitudinal gradient going around the corner.

The road is located in a residential subdivision so it is likely to be used mostly by cars and pickups. It should be reasonably motorable though for the odd service and emergency vehicle.

Thanks

 

[civilperson]

Use of both lanes is permitted to pass long wheel base trucks requiring 44' radius turns. Widening the lanes an extra two feet, from eleven to thirteen feet through the small radius curves is also a good idea, (sacrifice the shoulder width if necessary). Do not allow drive entrances at locations with less than 70' sight distance. Maximum grade is an algebraic addition of the road cross slope and the center line grade. Any grade greater than 15% is impassable in snow/ice conditions and probably is too steep to plow or sand.