HighwayDrainage Solutions

Highway drainage design is governed by multiple specifications: (1) AASHTO Drainage Manual provides overarching design guidance; (2) FHWA HEC-22 (Urban Drainage Design) covers inlet design and spacing; (3) State DOT specifications add local requirements; (4) AASHTO M306 governs casting materials and load ratings. Federal-aid highway projects must comply with FHWA requirements, while state and local roads follow respective DOT standards. Always verify current specifications with the project owner.

Highway drainage typically requires Class D (60,000 lbs) to Class E (100,000 lbs) ratings. Shoulder areas need Class D minimum for emergency vehicle access and maintenance equipment. Travel lanes require Class E where vehicles may track across inlets. Bridge deck drains often specify Class E due to concentrated traffic loading. All castings should meet AASHTO M306 for ductile iron or M105 for gray iron.

Bicycle-safe grate patterns prevent wheel entrapment and falls. FHWA and AASHTO recommend: (1) Slots no wider than 4" if parallel to travel; (2) Diagonal slots (45°) limit effective opening; (3) Reticuline (curved vane) patterns provide excellent bicycle safety; (4) Transverse bars at maximum 4" spacing. Many states mandate bicycle-safe grates on all roads with bicycle traffic. Common bicycle-safe products include Neenah R-3067 series and EJ catalog grates marked "bicycle safe."

Highway inlet spacing uses the rational method combined with gutter flow calculations per FHWA HEC-22: (1) Calculate runoff using Q = CIA (rational method); (2) Determine allowable spread (typically 8-10 ft from curb); (3) Calculate gutter capacity at allowable spread; (4) Space inlets so accumulated flow does not exceed gutter capacity. Computer programs like HY-12 simplify calculations. Typical results are 300-600 ft spacing depending on grade, cross-slope, and rainfall intensity.

Depressed highway medians require drainage to prevent ponding and hydroplaning hazards: (1) Slot drains along the median center for continuous collection; (2) Drop inlets at low points where slot drains outlet; (3) Minimum 2% longitudinal slope in median swales; (4) Outlet pipes crossing beneath travel lanes (often requiring jacking). Many DOTs now prefer raised medians with cable barriers, which eliminate median drainage but require edge drainage on both sides.

Highway shoulder drainage captures runoff before it reaches travel lanes: (1) Shoulder slopes 4-6% toward edge; (2) Edge drains (slotted pipe or French drain) along the shoulder/slope break; (3) Catch basins at low points collect edge drain flow; (4) Outlets connect to cross-drain culverts. Porous asphalt or permeable shoulders can reduce surface runoff. DOT specifications vary—some require positive drainage, others allow sheet flow to embankment slopes.

Bridge deck drainage has unique requirements: (1) Lightweight drain bodies to minimize deck load; (2) Scupper drains through deck for simple systems; (3) Trough drains along curbs for longer bridges; (4) Expansion joints that accommodate deck movement; (5) Freeze protection in cold climates. Discharge must avoid the waterway below—often piped to bridge ends. AASHTO LRFD Bridge Design Specifications govern bridge drainage design.

Highway drains face significant debris loading from roadside vegetation, tire debris, and sediment: (1) Use bar grates with 2-3" openings rather than perforated patterns; (2) Install debris hoods on curb inlets; (3) Size inlet capacity assuming 50% clogging; (4) Specify catch basins with sediment sumps (minimum 2 ft depth); (5) Establish regular inspection and cleaning schedules. Some DOTs require combination inlets (grate + curb opening) for debris tolerance.

Highway drainage castings are primarily ductile iron or gray iron per AASHTO specifications: (1) Ductile iron (AASHTO M306) for Class D-F applications—more impact resistant; (2) Gray iron (AASHTO M105) for lighter applications—more economical; (3) Steel grates where specified for certain DOTs; (4) Polymer concrete channels with iron grates for trench drains. Galvanized or coated options are specified in corrosive environments (de-icing salt exposure, coastal areas).

Highway drainage costs vary by type and location: (1) Standard catch basins: $2,000-5,000 installed per unit; (2) Curb inlets: $1,500-3,500 per unit; (3) Trench drains: $150-400 per linear foot installed; (4) Cross-drain culverts: $200-500 per linear foot depending on diameter. Costs increase significantly in rock excavation, high-traffic areas requiring traffic control, or urban settings with utility conflicts. State DOT bid tabs provide regional pricing benchmarks.