L . Underpass
An underpass is a passage way for a pedestrian or bicyclist that is typically below a bridge , roadway , or railroad . Underpasses can provide a safe connection between two land uses that are separated by a physical barrier . Roadways can provide a physical barrier due to high traffic volumes . Topographic conditions , safety , and other physical barriers can also warrant the construction of underpasses to help ensure pedestrian access and connections to occur .
As with underpasses or bridges , ADA – compliant ramps and / or stairs are used to effect the elevation change . It is found that pedestrians and bicyclists use grade separated crossings more often if the elevation change can be minimized or worked into the normal path of pedestrian and bicyclist movements . Underpasses can be more expensive than other bicycle and pedestrian solutions to install , but because they can be designed with less grade changes than overpasses , they are often preferable ( Figure 7.52.31 ).
Design Criteria :
• The minimum vertical clearance of an underpass is 10 feet . Special consideration shall be provided to maintain adequate clearance and a maximum grade of 5 percent for the approaching parkway . ( Figure 7.52 . 32 ).
• The minimum width is 12 feet .
• The underpass shall be wide enough to accommodate emergency vehicles .
• The underpass shall include adequate lighting for safety .
• The slope of ramps must be comply with ADA Guidelines and be accessible by all ages and abilities .
• When underpasses are used , approach grades shall slope away from the underpass to prevent flooding and provide positive drainage away from the underpass . Additional drainage measures may be required .
• Adequate sight distance should be provided in underpasses , preferably with open ends of the tunnel in clear view at all times .
• When designing an underpass , the aesthetic appearance of the facility shall be considered so that the structure fits within the surrounding context .
• Underpass materials can be either concrete or galvanized metal sheeting . Concrete is the preferred material .
• Pedestrian and bicycle access to roadway surfaces above underpass should be provided .
• When designing a tunnel , provide soil exploration to determine if a tunnel is feasible to construct .
• Tunnels should be designed to let more natural light in and provide a wide opening to be more inviting to pedestrians .
Figure 7.52.31 . A tunnel for multi-use trail underpass .
Figure 7.52.32 . Typical underpass / undercrossing configurations .
Chapter 7 : Design Guidelines | 7-45