Published
When the crossing needs a bridge: HS-20 trucks, three feet four inches of extra width, and the 100-year flood underneath.
When the crossing stops being a culvert job
Most dirt work in this county is culvert work. But a crossing over a live stream, a canal, or a steep draw is a structure, and Appendix B section 1.14 is the county’s written standard for it. The section opens by putting the whole subject in an engineer’s hands: bridge structures shall be designed by an Idaho registered professional engineer in accordance with AASHTO bridge specifications, latest edition.
The first thing the code settles is what the bridge must survive:
- The minimum design vehicle is an AASHTO HS-20 truck for local roads.
- Collector and arterial roads are designed to an HS-25 truck.
- Bridges may instead be designed to the AASHTO LRFD manual’s HL-93 loading.
Those letters are worth being able to read in a bid. An HS-20 is a three-axle semitruck; the HS-25 is the same truck a quarter heavier. A bridge quoted to “HS-20” on a collector is quoted to the wrong truck for its road.
The width rule: the bridge is wider than the road it carries
Section 1.14.030 fixes the clear width between curbs or rails:
“The minimum width of a bridge structure from the face-to-face of curb or the face-to-face of the guardrail or bridge rail shall be the full width of the approach roadway including pavement width and shoulder width, plus one foot eight inches (1'-8") on each side, for a total additional width of 3 feet 4 inches (3'-4").”
— Fremont County Development Code, 2011 Edition (Ordinance 2011-04), Appendix B, section 1.14.030. Consolidated through Ordinance 2026-03. Retrieved 7 September 2026
The bridge is three feet four inches wider than the road, by rule. A 30-foot approach roadway — the county’s standard subdivision cross-section — therefore arrives at a bridge at least 33 feet 4 inches clear. The reason a bridge is wider than its road is the same reason the road itself carries shoulders: the edge of a structure is the one place a driver cannot be allowed to run out of road.
And the approach to the bridge has its own geometry: there shall be a minimum 50-foot long, 90-degree approach to all bridges. Fifty feet of straight, square road before the deck — no curve, no skew — so that a truck arrives at the rail aligned with the span.
Water: two feet over the 50-year flood, and the 100-year passes underneath
The clearance rule is the cleanest sentence in the section:
- The vertical clearance above waterways shall be two feet above the 50-year flood.
- The 100-year flood must pass beneath the bridge.
- Vertical clearance over other roadway surfaces is a minimum of 16 feet.
Read together: the deck sits above the half-century storm with two feet to spare, and the full-century storm — the deeper one — still fits under the structure rather than against it. The Public Works Director will require a hydraulic analysis and drawings, including slope and cross-sections of the waterway, before approving the bridge. And the code adds a clause no lowland county needs: ice flow conditions can affect clearance and shall be taken into consideration by the designer. In this county that is not a footnote; spring ice on a mountain stream moves like a plow blade.
Canals, fish, and the two approvals outside the county
Two provisions hand part of the decision to authorities other than Fremont County:
- Any crossing over a canal must be approved by the canal company as to type of structure, clearance, and any other criteria the company finds pertinent.
- An applicant may be required to install a bridge or other open-bottom structure rather than a culvert on any natural watercourse, on the recommendation of the Idaho Department of Fish and Game and with the Public Works Director’s approval, where required to protect the fishery.
The Fish and Game clause is the one that turns a cheap crossing into an expensive one. An open-bottom structure preserves the streambed — and a recommendation from the department can arrive on any natural watercourse, not only famous trout water. Budgeting a crossing without knowing whether the stream is a fishery question is how a bid gets rewritten mid-project.
Materials and retaining walls
The code is short and restrictive about what structures may be made of:
- Only structures of steel, steel and concrete, or treated wood may be used; anything else is approved or disapproved by the Public Works Director case by case.
- Retaining walls should be reinforced concrete, bin walls, reinforced earth, or concrete crib walls, with the same case-by-case approval for anything else.
- Every retaining wall structure must be designed by an Idaho registered professional engineer and approved by the Public Works Director before construction.
The retaining-wall clause reaches farther than bridges. Cut-and-fill road building on the slopes this county allows generates walls routinely, and the code puts each one under an engineer’s stamp — a sibling rule to the slope thresholds that decide where a road or building may go at all. A crib wall assembled from a catalog sheet by the equipment operator is not a wall this code recognizes.
The numbers, in one place
| Requirement | The standard |
|---|---|
| Design vehicle | HS-20 local / HS-25 collector and arterial, or HL-93 |
| Clear width | Approach roadway width + 1’-8” per side (3’-4” total) |
| Approach | 50 feet long, 90 degrees, at every bridge |
| Waterway clearance | 2 feet above the 50-year flood; the 100-year passes beneath |
| Clearance over roadways | 16 feet minimum, ice flow considered |
| Materials | Steel, steel and concrete, or treated wood |
What to settle before anyone prices a crossing
- Establish whether the crossing is a culvert or a structure — the drainage floors cover pipe; this section covers everything above them.
- Identify the road class, because the design truck follows it: HS-20 on a local road, HS-25 on a collector or arterial, or HL-93 under the LRFD manual.
- Size the width from the approach roadway: full road and shoulder width, plus one foot eight inches each side.
- Reserve a 50-foot straight, 90-degree approach at each end of the span.
- Get the hydraulic analysis started early — the Director requires it, with waterway slope and cross-sections, before approval.
- Ask two questions outside the county: does a canal company control the channel, and has Fish and Game flagged the stream — either can change the structure type.
- Put every retaining wall under an engineer’s design before it is built, and stick to the approved material lists.
A structure is the small share of road work that is mostly engineering and permits rather than moving dirt — which is precisely why the county wrote its other right-of-way structures to the same pattern of drawings, approval, and inspection. The dirt part still has to be done right; it is just no longer the part that decides anything.
Common questions
What truck must a county bridge be designed for?
An AASHTO HS-20 truck on local roads and an HS-25 truck on collector and arterial roads. Bridges may also be designed to the AASHTO LRFD Bridge Design Manual's HL-93 loading.
How wide must a bridge be compared to its road?
The minimum width face-to-face of curb or rail is the full width of the approach roadway, pavement and shoulders included, plus one foot eight inches on each side — three feet four inches of additional width in total.
What approach geometry does a bridge require?
A minimum 50-foot long, 90-degree approach to all bridges — a straight, square stretch of road at each end of the span.
How high must a bridge sit above the water?
The vertical clearance above waterways must be two feet above the 50-year flood, and the 100-year flood must pass beneath the bridge. A hydraulic analysis with waterway slope and cross-sections is required before approval.
What is the vertical clearance over a roadway?
A minimum of sixteen feet over other roadway surfaces, with ice flow conditions taken into consideration by the bridge designer.
Who approves a crossing over a canal?
The canal company. Any crossing over a canal must be approved by the canal company as to type of structure, clearance, and any other criteria it determines to be pertinent.
Can the county make me build a bridge instead of a culvert on a stream?
Yes. On the recommendation of the Idaho Department of Fish and Game and with Public Works approval, an applicant may be required to install a bridge or other open-bottom structure on any natural watercourse to protect the fishery.
What materials may a bridge or retaining wall use?
Structures of steel, steel and concrete, or treated wood; retaining walls of reinforced concrete, bin walls, reinforced earth, or concrete crib walls. Other materials are considered by the Public Works Director case by case.
Do retaining walls need an engineer?
Yes. All retaining wall structures must be designed by an Idaho registered professional engineer and approved by the Fremont County Public Works Director prior to construction.