Published
What a county road is made of: 18 inches of pit run under 4 inches of crushed.
The county publishes the recipe, layer by layer
Ask what a road is made of and the honest local answer is usually “whatever the operator had.” But Fremont County writes its answer down. Appendix B section 1.12 of the Development Code fixes the cross-section of a road built to county standard, from the shape of the surface down through the gravel, and it is short enough to read in a minute.
Two provisions carry the whole section:
“A minimum sub-base depth of 18 inches of pit run gravel at a width of 30 feet, compacted to 95% of standard density is required on all roads. The pit run gravel should be placed in layers not exceeding 9 inches and compacted between layers.”
— Fremont County Development Code, 2011 Edition (Ordinance 2011-04), Appendix B, section 1.12.020. Consolidated through Ordinance 2026-03. Retrieved 7 September 2026
Eighteen inches, thirty feet wide, ninety-five percent, in nine-inch lifts. That is the load-bearing half of every county-standard road in this valley, and each number answers a specific failure. The lifts matter as much as the total: gravel dumped in one eighteen-inch pile and rolled once does not reach the density the same gravel reaches in two compacted layers, and the code says so outright.
The surface: thirty feet, built from a 24-foot traveled way
Section 1.12.010 sets the minimum width of the roadway surface for a two-lane road in a platted subdivision at 30 feet — and it decomposes that number: a 24-foot wide traveled way with a 3-foot shoulder on each side. Additional lanes, where they exist, must be a minimum of 12 feet wide unless the Public Works Director specifically approves less.
Two qualifiers keep the width honest:
- Roads that are or will be maintained by Fremont County may be wider than 30 feet.
- The width of county-maintained roadways is set by the Public Works Director case by case, not by a single number.
That pair of clauses is why two roads in the same subdivision can legitimately be built to different widths. The 30-foot figure is the floor for a platted subdivision road, not a promise of uniformity — and the width question is also a winter question up here, because a road sized in August still has to function with snow stored beside it in February.
The gravel sandwich, in order
Read top down, the code’s layers are:
| Layer | The standard |
|---|---|
| Sub-base | 18 inches of pit run gravel, 30 feet wide, compacted to 95% of standard density, in lifts not exceeding 9 inches |
| Surface gravel | 4 inches of compacted three-quarter-inch crushed gravel over the sub-base |
| Asphalt, if required | Minimum 24 feet wide at a minimum depth of 2.5 inches |
| Gravel roadway shape | A 4% cross-slope so the surface sheds water |
The three-quarter-inch crushed surface is the same base-and-surface logic a private driveway runs on, written at county scale: pit run underneath for structure, crushed and compacted on top for a crust that sheds water. The sub-base can legally be thinner than 18 inches, but only one way:
- A depth of less than 18 inches may be accepted if it is designed by an Idaho registered professional engineer and the design is accompanied by a geotechnical engineering report certifying the reduced depth.
- The Public Works Director can require more than 18 inches based on knowledge of the soils at the location.
So the 18-inch figure is a default with an escape hatch downward through an engineer and an override upward through the Director. Where a bid quotes “12 inches of gravel, plenty for cabins,” the code is the document that quote is quietly disagreeing with — and gravel volume is exactly where excavation bids lose their comparability fastest.
Asphalt has its own conditions
Where asphalt is required, the paving is not only width and depth. Four conditions come with it:
- New roads that intersect an existing paved road must be paved to the right-of-way line of the existing road.
- Intersections carry a minimum 30-foot radius where the two pavements meet.
- The asphalt must meet or exceed Fremont County pavement mix requirements — a copy of the mix design can be obtained from the Road and Bridge Department.
- A certified materials testing company must conduct the mix design, temperature, and compaction tests, and the results go to Fremont County Public Works.
If curb and gutter is installed, it must follow the current edition of the Idaho Standards for Public Works Construction, and the Director reviews and approves the curb type before construction.
The grade rule where a road meets another road
Section 1.12.060 fixes the first stretch of every access approach where it meets the pavement:
- The first 10 feet of all access approaches must have a decline of 3% in slope, measured from the edge of pavement or the shoulder of a gravel road.
- The next 50 feet can rise or fall, but by no more than 3%.
The reason is drainage at the joint — water leaving the road surface rather than running down the approach. It sits beside a separate, older rule about the last fifteen feet of a driveway, which belongs to the county’s access standard and is covered there.
What this means for anyone building road
- Price the sub-base as a volume, not a line item: 18 inches across a 30-foot width is a known quantity of pit run per running foot, and it is the biggest gravel number in the bid.
- Hold the operator to 9-inch lifts with compaction between them — the code names the practice, not just the total depth.
- Confirm whether Public Works expects more than 18 inches on your soils before the bid, because the Director can require it after.
- Specify three-quarter-inch crushed for the surface course, and compaction, or the crust never forms.
- Check whether any segment requires asphalt — and remember an intersection with an existing paved road pulls paving, a 30-foot radius, and certified testing into the scope.
- Lay the first 10 feet of every approach at a 3% decline off the road, then keep the next 50 feet inside 3% either way.
- Shape gravel surfaces to a 4% cross-slope so the road sheds water instead of carrying it.
None of this is exotic — it is a written recipe for the most ordinary object in the county. What makes it worth reading before the machines arrive is that the road built to it is the road the county will accept, and the acceptance process, from the drawings of record to the one-year warranty, measures what was actually placed against exactly these numbers.
Common questions
How wide must a subdivision road be in Fremont County?
The minimum roadway surface for a two-lane road in a platted subdivision is 30 feet: a 24-foot traveled way with a 3-foot shoulder on each side. Additional lanes must be at least 12 feet wide unless the Public Works Director approves less.
How deep is the gravel under a county-standard road?
A minimum sub-base of 18 inches of pit run gravel, 30 feet wide, compacted to 95% of standard density, placed in layers not exceeding 9 inches and compacted between layers.
Can the sub-base be thinner than 18 inches?
Only if an Idaho registered professional engineer designs the reduced depth and a geotechnical engineering report certifies it. The Public Works Director can also require more than 18 inches based on the soils at the location.
What goes on top of the sub-base?
A 4-inch depth of compacted three-quarter-inch crushed gravel is required over the sub-base. A gravel-surface roadway carries a 4% cross-slope so it sheds water.
How thick does asphalt have to be on a county road?
Where asphalt is required, the pavement must be a minimum 24 feet wide with a minimum depth of 2.5 inches, meeting Fremont County pavement mix requirements, with mix, temperature, and compaction testing by a certified materials testing company.
What happens where a new road meets an existing paved road?
The new road must be paved to the right-of-way line of the existing road, with a minimum 30-foot radius at the intersection.
What grade must a road meet an access approach at?
The first 10 feet of all access approaches must decline 3% from the edge of pavement or gravel shoulder, and the next 50 feet can incline or decline by no more than 3%.
Does curb and gutter have its own standard?
Yes. Curb and gutter must follow the current edition of the Idaho Standards for Public Works Construction, and the Public Works Director reviews and approves the curb type before construction.
Why do the 9-inch lifts matter if the total is the same?
Because compaction is achieved per layer. Pit run gravel placed in lifts not exceeding 9 inches and compacted between layers reaches the required 95% of standard density; the same gravel dumped in one thick layer and rolled once does not.