Why is ACHD's standard different from the city's?

Because ACHD owns the right-of-way for local and collector roads in Ada County, not the city. The city subdivision code regulates lot layout, density, and use; ACHD regulates the road itself. Both apply, and where they differ, ACHD wins on anything inside the right-of-way.

Can I use private roads instead of public?

Sometimes. Private roads avoid the public right-of-way requirement, which gives the developer more flexibility on width, design, and maintenance. The tradeoff: the HOA owns and maintains the road, which is sometimes viewed by buyers as a burden. For dense, high-value subdivisions, private roads can be a competitive advantage. For typical workforce housing, public roads are the default.

What's the difference between a local street, a collector, and an arterial?

Hierarchy of road function. Local streets serve individual lots and have low traffic volumes. Collectors carry traffic from local streets to arterials. Arterials are higher-speed, higher-volume roads (often state routes managed by ITD). Each level has progressively wider sections, more pedestrian infrastructure, and stricter ACHD or ITD requirements.

How much does road design cost as part of a project?

Highly variable. Small infill subdivisions may need only intersection improvements and a short road extension. Large master-planned communities may require multiple miles of internal road plus offsite improvements to adjacent collectors. Bailey scopes road design as part of the overall civil engineering package — not as a separate fee.

What if ACHD requires offsite improvements?

Common for projects with significant traffic impact. ACHD may require road widening, intersection improvements, signalization, or new turn lanes at adjacent intersections. The cost can be substantial — tens to hundreds of thousands of dollars per intersection. Identify ACHD requirements during the feasibility phase, not during engineering. The cost of offsite improvements is a key input to the project pro forma.

Do I need a traffic study?

Depends on project size and trip generation. Most large residential subdivisions and commercial projects need a traffic impact analysis. Smaller projects may need a simpler trip generation memo. ACHD or the relevant city's planning department will tell you what level of analysis is required.

Can I reduce parking minimums in Boise's Modern Zoning Code?

Yes — the Modern Zoning Code reduced or eliminated parking minimums in several zones, particularly in mixed-use and transit-supportive areas. Verify your specific zone before designing. Reducing parking can free up significant land area for other uses on tight sites.

What's the best way to coordinate with ACHD early?

A pre-application meeting with ACHD project review staff during the feasibility phase. ACHD has formal pre-application processes and dedicated project managers. Use them. Bailey coordinates ACHD engagement as part of every Ada County project — it's the single most reliable way to compress the entitlement timeline.

What road and parking standards does ACHD require for new subdivisions?

Who decides road standards in your city

In the Treasure Valley, road standards are set by two authorities, not one: the highway district that owns the road, and the city that approves the subdivision.

ACHD (Ada County Highway District) owns and maintains local and collector roads in Boise, Meridian, Eagle, Garden City, Kuna, and Star. ACHD has its own design standards that govern lane widths, cross sections, curb types, intersection design, sight distance, and bicycle/pedestrian facilities. These standards apply regardless of what the city wants — ACHD has independent authority over its rights-of-way.

Canyon County has a different structure. Notus/Parma Highway District 2 covers Notus, Parma, and surrounding areas. Other parts of Canyon County are managed by Canyon County itself or by smaller districts. Standards vary, and the relevant district must be coordinated separately from city planning.

ITD (Idaho Transportation Department) owns state highways and federal routes (SH-, US-, I-). Any project with access onto a state route or near one needs ITD coordination on top of the highway district and city.

The most consequential thing to understand: ACHD’s standards differ from city subdivision codes in ways that aren’t obvious. Kuna and Star require 36-foot back-of-curb-to-back-of-curb local streets versus 33 feet in most other Ada County cities. That extra 3 feet of pavement, multiplied by every linear foot of street in the subdivision, is real money — both in pavement cost and in the lot frontage that gets eaten by the wider section.

Roadways are the largest single line item in most residential land development budgets. Get the section wrong and the project economics shift before you’ve drawn a single lot.

Bailey methodology

The street section — width, curbs, sidewalks

The street section is the cross-section view of a road: pavement, curb, gutter, sidewalk, planter strip, right-of-way line. Every element is a choice with cost and yield implications.

Pavement width

Most Treasure Valley municipalities have standard residential street widths of 30 to 36 feet of pavement (back-of-curb to back-of-curb) within a 50- to 60-foot right-of-way. A typical residential street improvement runs $150 to $200 per linear foot at 30 feet wide. Wider sections increase that cost roughly proportionally.

The single largest cost-saving design move on most projects is reducing pavement width to the minimum allowed. Narrower roads reduce:

Pavement and base material cost
Right-of-way requirements (more lot, less street)
Clearing and earthwork
Stormwater management requirements (less impervious surface = smaller basins)
Speed of vehicles in the subdivision (a traffic calming benefit the community usually appreciates)

The catch: not every city allows narrower sections. ACHD has specific minimum standards, and some city codes are conservative. The conversation with planning staff and the highway district happens at the pre-application stage — not at the comment-response cycle.

Curb type

Treasure Valley subdivisions typically use one of three curb types:

Standard L-block barrier curb — vertical face, 6-inch height. The conventional default. Highest material cost, clearest lane delineation, hardest on errant vehicles.
Concrete mountable curb — sloped face, 4-inch height. Cheaper than L-block, allows occasional vehicle mounting (parking, deliveries), preferred in some HOA-maintained subdivisions.
Asphalt rolled curb — least expensive, integrated with the pavement, lowest profile. Acceptable in some Treasure Valley cities for lower-volume residential streets.

The choice interacts with drainage: barrier curb requires inlet structures at low points; rolled curb can shed water to roadside swales without inlets at all.

Sidewalks and pedestrian infrastructure

Most Treasure Valley residential cities require sidewalks on at least one side of every local street, often both. Width is typically 4 to 6 feet, with a 4- to 8-foot planter strip between the curb and the sidewalk. The planter strip improves the streetscape, accommodates street trees, and provides snow storage.

For higher-volume corridors and commercial sites, ACHD or the relevant city may require detached sidewalks, ADA-compliant ramps at every intersection, and separated bike facilities. Verify the standards for your specific corridor early — retrofitting pedestrian infrastructure into a finalized design is expensive.

Roadside swales as an emerging alternative

A growing Treasure Valley trend: replacing curb-and-gutter with roadside swales. The swale handles drainage, eliminates inlet costs, reduces impervious surface, and shifts the maintenance from a pipe-cleaning operation to a vegetation-cutting operation. The buying public has been slow to accept swale sections (the conventional curb-and-gutter look feels “complete” to most buyers), but the regulatory direction in Boise’s Modern Zoning Code and several other Treasure Valley codes points toward more swales and less concrete.

Alignment and earthwork

The vertical and horizontal alignment of subdivision roads should complement the topography of the land. Road alignments should follow the terrain by being parallel or perpendicular to the contour lines as much as possible. It’s preferable to design roadways to be cut rather than filled, so the lots are automatically higher than the roadway. This:

Facilitates proper lot drainage (water flows away from houses, toward the street)
Allows the maximum number of trees to be saved
Reduces import/export of fill material

A well-aligned road network keeps cuts and fills small. Large cuts and fills are expensive — they require staging areas for excavated material, additional clearing, and multiple equipment passes. The most efficient earthwork balances cuts and fills on the site so material doesn’t have to be hauled in or out.

If the site has material that can be used for road base — gravel, rock, aggregate — and the material is permitted by the local municipality, that further reduces project cost. Verify with the highway district before assuming.

Parking design choices

For commercial, multifamily, and mixed-use sites, parking design has its own set of decisions:

Parking ratios

Each city sets minimum parking ratios per use type. A typical multifamily project requires 1.5 to 2 spaces per dwelling unit. Commercial uses vary widely — retail at 4 spaces per 1,000 square feet, office at 3 to 4, restaurant at 8 to 10. The trend across the Treasure Valley is reducing parking minimums, particularly in transit-supportive corridors and infill sites. Boise’s Modern Zoning Code has reduced or eliminated parking minimums in several zones — verify your specific zone’s requirement before designing.

Surface vs. structured parking

Surface parking is cheap to build and expensive in land area. Structured parking (above-grade or underground) is expensive to build and efficient in land area. For high-density urban sites, structured parking is often the only way to make the math work. For most Treasure Valley residential and small commercial sites, surface parking is the default.

Stormwater interaction

Parking lots are large impervious surfaces. Their stormwater contribution often drives the size of the project’s detention or retention facility. Reducing parking minimums reduces stormwater requirements, which reduces basin size, which frees up land for other uses. The cascade matters more on tight sites than on sprawling ones.

ADA and accessibility

Every parking lot must meet ADA requirements: accessible spaces (1 per 25 standard spaces, with at least 1 van-accessible per 8 accessible), 8-foot-wide standard accessible spaces with 5-foot access aisles, accessible routes from parking to building entrances. Get the ratios right in the original design — retrofits are expensive and disruptive.

Cost tradeoffs that matter

A few decisions consistently move the needle on project cost:

Narrower streets vs. standard: A 3-foot reduction in pavement width across 2,000 linear feet of street saves roughly $9,000 to $12,000 in pavement cost alone, plus the lot yield benefit of recovered frontage.

PVC vs. ductile iron water mains: PVC is substantially cheaper than DIP for water distribution. If the local jurisdiction allows it, use it. If they don’t, document and request — longevity, maintenance, and ease of installation are PVC’s positive attributes.

Maximize manhole spacing: Sanitary sewer designed at the minimum-pipe-diameter allowed (often 6-inch with cleanouts in lieu of 8-inch with manholes), with the maximum allowed distance between manholes (typically 400 feet), can save tens of thousands on a typical subdivision. Use easements on lots inside horizontal road curves where needed.

10-year vs. 25-year storm design: Many municipalities require 25-year storm design for residential drainage systems. A 10-year storm design is adequate for housing developments in most contexts, and the cost savings are meaningful. Verify the local requirement and request an alternative if the regulatory standard is conservative.

Joint trenching: Coordinating water, sewer, gas, and electric in shared trenches reduces excavation costs and conflicts. Coordinate with utility providers early — late changes to one utility’s location create cascading impacts.