By Kjel Jones, independent excavation & dirt-work researcher for Island Park · licensed Idaho real estate agent, LPT Realty

Published 2026-08-26

The state sets a minimum distance between driveways — and on a rural highway it is 650 feet

The rule that decides whether a lot gets its own driveway

Width and radii are negotiable geometry. Spacing is the one that says no. If the nearest existing access is closer than the published minimum, the approach you want may not be permittable at that location at any width — and no amount of gravel changes it.

The distances are in Table 1 of IDAPA 39.03.42. The approach permit page covers the permit itself and the width page covers the geometry. This is the constraint that comes before either.

Two variables select your row: the highway type — the state's own functional designation of the route, which is not something you can read off a sign — and the area type, rural through urban. Ask the district office in Rigby which row your frontage falls in before you assume.

Table 1, as published

All distances in feet. Column (A) is public road spacing; (B) is a driveway upstream from a public road intersection; (C) is a driveway downstream from an unsignalized public road intersection; (D) is the distance between unsignalized accesses other than public roads — the column most private driveways are judged on.

Starred figures carry a condition: where the public road or private access intersection is signalized, they apply to driveways restricted to right-in / right-out movements only, and for unrestricted driveways the minimum is 500 feet from a signalized intersection. Interstates and freeways are accessible only by interchanges, and expressways only at locations the Department specifies.

The rule attaches an important qualifier to the whole table:

Distances in table are minimums based on optimal operational and safety conditions such as adequate sight distance and level grade.

— IDAPA 39.03.42, Table 1 footnote (adminrules.idaho.gov, retrieved 2026-08-17)

Optimal conditions are the assumption, not the finding. A rise, a curve, a grade or a snow bank is not an optimal condition, and the rule gives the District Engineer authority to deny a permit or require a Traffic Impact Study when an on-site review shows those assumptions do not hold.

What 650 feet looks like on the ground

Six hundred and fifty feet is roughly an eighth of a mile. On frontage of less than that, an approach placed anywhere may sit inside the minimum from a neighbour's existing access — and the table's rural rows are the tightest constraint in the whole document precisely because rural frontage is where people expect the fewest rules.

• Frontage shorter than the spacing minimum does not automatically fail, but it does mean the location is decided by the neighbours' accesses rather than by where you want the house.
• A joint-use approach is the usual answer — one permitted access serving two properties. It is explicitly contemplated by the rule, and it carries a wider minimum width.
• An existing access is an asset. If ground already has a permitted approach in a workable place, that is worth real money and is worth confirming in writing before closing.
• Access rights may already be limited. The review expressly considers current access control requirements and deed restrictions, neither of which is visible from the shoulder.

The exception, and the study that comes with it

The District Engineer can approve less than the table's minimums, but the rule prices the exception:

For any exception that would result in a decrease in access spacing of more than ten percent (10%) of the distances set forth in Table 1, a Traffic Impact Study will be required in order to determine whether auxiliary lanes or other appropriate mitigation must be included in the permit's conditions.

— IDAPA 39.03.42.400.03.e (retrieved 2026-08-17)

So there is a small tolerance and then a cliff. A reduction inside ten percent can be justified and documented; beyond it, a study is required, and the study may conclude that a turn lane is the condition of your driveway. The rule says the basis for any exception must be justified and documented, and it names overriding economic opportunity considerations as something that may form part of that basis.

A study is also required, unless the District Engineer waives it in writing, when a new or expanded development seeks direct access and at full build-out will generate 100 or more new peak-hour trips, or 1,000 or more vehicles per day, or meets the rule's threshold values:

Those are subdivision and commercial numbers, not cabin numbers — which is the useful half of the finding. A single residence is nowhere near any threshold, so if a study is being discussed on a one-house approach, it is coming from the spacing exception or an on-site review, not from trip generation.

When a study is required, the rule says it documents access needs and impacts and whether highway modifications are necessary. It names the modifications it has in mind, and they are worth reading as a list of what an exception can cost:

• Turn lanes, and acceleration or deceleration lanes — construction in the state's travel way, at the applicant's expense
• Additional through lanes — the largest item on the list by some distance
• Medians, which can convert a two-way approach into right-in / right-out
• Traffic signals, subject to the Department's own warrant requirements
• Removal or consolidation of existing approaches — possibly including one you already use
• Approaches limited to right-in / right-out movements, which changes how you leave your own property

How to check spacing before you buy or build

1. Measure the frontage and the distance to the nearest access on either side, including field and neighbour approaches, before anything else.
2. Ask the Rigby district office for the highway type and area type on your stretch. Those two answers pick the row, and you cannot derive them from a map.
3. Compare against column (D) for a private driveway, and against (B) and (C) if a public road intersection is nearby.
4. If you are short, ask about joint use before asking for an exception — it is the cheaper of the two and does not risk a Traffic Impact Study.
5. Get any answer that matters in writing, and remember a verbal denial is the rule's default; only a written denial can be appealed.

Re-verify before relying on any figure here. This page was written from the current published text of IDAPA 39.03.42 retrieved on 2026-08-17. The rule is free to read at adminrules.idaho.gov.

Common questions

How far apart do driveways onto a state highway have to be?

On a rural statewide or regional route the published minimum between unsignalized accesses other than public roads is 650 feet. On a rural district route it is 500 feet. Urban and transitional rows are shorter, down to 250 feet in some signalized cases.

What is public road spacing?

It is the minimum distance between public road intersections on the highway, published separately from driveway spacing. On a rural statewide route it is 5,280 feet — a full mile. On a rural regional route it is 2,640 feet.

How do I know which row applies to my frontage?

Two things select it: the highway type, which is the state's functional designation of the route, and the area type, rural through urban. Neither can be read off a sign, so ask the district office in Rigby which row your stretch falls in.

My frontage is shorter than 650 feet. Can I still get an approach?

Possibly. The distances are minimums assuming optimal conditions, and the District Engineer can approve a decrease if the basis is justified and documented. A joint-use approach shared with a neighbour is the other common route, and it does not trigger a study.

When is a Traffic Impact Study required?

For any exception decreasing spacing by more than ten percent of the Table 1 distances. Also, unless waived, when a development seeks direct access and at build-out generates 100 or more new peak-hour trips, 1,000 or more vehicles per day, or meets the rule's size thresholds.

Would a single cabin ever trigger a Traffic Impact Study?

Not on trip generation. The thresholds are 100 residential units, 35,000 square feet of retail, 50,000 of office or 70,000 of industrial. If a study is raised on a one-house approach it is coming from a spacing exception or an on-site review instead.

Do these distances guarantee approval if I meet them?

No. The table's own footnote says the distances are minimums based on optimal conditions such as adequate sight distance and level grade. The District Engineer may deny a permit or require a study where an on-site review shows those conditions do not exist.

What does a Traffic Impact Study actually decide?

It documents access needs and impacts and whether highway modifications are needed for the new traffic. The rule gives examples: turn lanes, additional through lanes, acceleration or deceleration lanes, medians, signals, removal or consolidation of existing approaches, and right-in/right-out restrictions.

Does any of this apply to a county road?

No. IDAPA 39.03.42 governs encroachments on state rights-of-way. An access onto a Fremont County road is a county matter and is covered on the who-pulls-the-permit page.