Route Selection

Part of Roads and Transport

Choosing the best path through terrain by evaluating obstacles, drainage, gradient, and material availability.

Why This Matters

The difference between a good road route and a bad one is often not distance — it is the quality of the terrain the road passes through. Two routes of equal length can require completely different amounts of construction labor, experience completely different drainage situations, and have completely different seasonal usability.

The principles of route selection were understood by early road engineers almost as soon as people began building roads. Animals instinctively find the paths of least resistance. Human planners learned to read the same terrain cues — valley floors for level grades, ridgelines for good drainage, saddles for mountain crossings — and turn them into deliberate route choices.

The goal of route selection is to find the path that satisfies your requirements (load capacity, seasonal usability, distance) while minimizing total construction effort and ongoing maintenance. Sometimes a route that adds 20% in distance eliminates 80% of the construction cost by avoiding a river crossing or a wet valley floor.

The Five Route-Finding Principles

Principle 1: Use Natural Landforms as Highways

Nature has already done much of the route-selection work. Several landforms serve naturally as road corridors:

Ridgelines: High ground between two valleys. Ridgeline roads have natural drainage (water runs off both sides), firm footing (ridges are often rocky or well-drained), and good visibility. The disadvantage is that they require constant climbing and descending to reach valley settlements.

Valley floors: The flat land alongside rivers and streams. Valley roads minimize elevation change and were the routes followed by most pre-modern trade roads. The disadvantage is flood risk and wet ground near the water.

Terraces above flood plains: The best of both worlds — follow the flat valley but elevated above flood level. Most natural river valleys have old river terraces (slightly raised flat land a few meters above the active floodplain) that are both level and dry.

Saddles (mountain passes): Where ridges are interrupted by lower ground. Always use the lowest available saddle for a mountain crossing, even if it adds lateral distance.

Principle 2: Minimize Stream Crossings

Every stream crossing is a construction cost, a maintenance liability, and a bottleneck. A road that crosses five streams requires five bridges or fords; a slightly longer route that crosses only one saves enormous construction labor.

Evaluating a crossing:

• Width and depth: Wide, deep crossings need bridges. Narrow, shallow crossings can be forded.
• Flow variability: Does this stream flood seasonally? A ford that is passable 10 months of the year and impassable for 2 months (when you most need to move harvest goods) is a serious problem.
• Bank conditions: Firm, stable banks allow easy road approaches. Soft or eroding banks require heavy reinforcement.
• Alternatives: Is there a narrower crossing point within a reasonable distance?

Crossing priority hierarchy:

1. Rocky natural ford (no construction needed, reliable)
2. Narrow crossing point suitable for a simple log bridge
3. Normal crossing requiring a proper bridge
4. Wide, deep crossing — route around if possible

Principle 3: Avoid the Wet Ground

Soft ground — marshes, bogs, wet meadows, seeping slopes — is the enemy of roads. A road built on soft ground settles, deforms, and requires constant maintenance. Wet soil has far lower bearing capacity than dry soil.

Identifying wet ground in the field:

• Vegetation indicators: cattails, rushes, sedges, willows, alder trees all indicate consistently wet soil
• Soil indicators: dark organic soil, blue-gray gleyed soil, soil that smells of sulfur or decay
• Terrain indicators: flat low ground with no visible outlet drainage, concave hollows in slopes (water collects here), ground that springs back when stepped on

Options when wet ground cannot be avoided:

• Route around it: Even a significant detour may be cheaper than crossing wet ground
• Follow the edge: Stay on the drier soil at the margin of the wet area rather than crossing through it
• Corduroy construction: If crossing is unavoidable, build a corduroy (log-and-gravel raft) over the wet section

Principle 4: Keep Grades Within Limits

Maximum sustainable grade for loaded wheeled vehicles is approximately 8-10%, with 5% as the comfortable working limit.

Reading terrain for grades:

• Contour spacing on maps: closely spaced contours = steep; widely spaced = gentle
• In the field: if you are breathing hard while walking at a normal pace, the grade exceeds 8-10%
• Use a simple clinometer (plumb bob on a protractor face) or water level to measure grades precisely before committing to a route

Grade reduction techniques:

• Follow the contour (traverse the slope rather than going straight up)
• Use switchbacks for unavoidable steep sections
• Take a longer valley route to gain elevation gradually rather than ascending a direct but steep path

Principle 5: Use Available Materials

A road that can be built from materials immediately at hand is far cheaper than one requiring long-distance haul of construction materials.

Inventory the materials along each candidate route:

• Rock outcrops that can be quarried for base and surface stone
• Natural gravel deposits (river gravels, glacial deposits)
• Clay deposits for binding fines
• Dense forest timber for corduroy, bridges, or culverts

The haul distance rule: Moving construction materials more than 2-3 km from source to road multiplies the cost dramatically. If Route A has abundant gravel along its length and Route B requires all gravel to be hauled 5 km, Route A may be preferable even if it is longer.

Reading Terrain Indicators

Water Flow Indicators

Read the land surface to predict where water flows:

• Flat or convex ground sheds water (dry conditions)
• Concave or bowl-shaped ground collects water (wet conditions)
• Water marks on trees and posts show flood levels
• Soil color changes (dark staining high on banks) indicate past flooding
• Old channel patterns (oxbows, meander scars) show where rivers have flooded historically

Soil Stability Indicators

• Hard, stony soils = good load-bearing capacity
• Soft, plastic (moldable) soils = poor load-bearing capacity
• Sand below water table = will shift under load
• Organic (dark, spongy) soil = very poor, must be removed or avoided
• Rock at or near surface = excellent bearing capacity, difficult to excavate

Vegetation Indicators

Vegetation Type	Soil Implication
Oak, hickory, locust on slopes	Dry, stable, often rocky
Alder, willow along streams	Wet, soft, flood-prone
Cattails, rushes in flat areas	Permanent wetland
Sedges in meadows	Seasonally wet
Mossy, spongy ground	High water table
Dry prairie grass	Well-drained, good bearing

Making the Final Choice

After field reconnaissance, you typically have two or three viable route options. Compare them on these criteria:

Hard constraints (eliminate routes that fail these):

• Maximum grade exceeds 10%
• Requires crossing a major river without possible bridge site
• Passes through permanent wetland with no practical corduroy option
• No stone or gravel available within 3 km for base material

Soft factors (rank remaining options):

1. Total construction labor (estimate days of work)
2. Annual maintenance effort (more crossings and wet sections = more maintenance)
3. Seasonal reliability (routes through flood-prone areas may be impassable when most needed)
4. Route length (distance traveled by all users over the lifetime of the road)

Select the route with the lowest total lifetime cost — construction plus maintenance over 20 years — that meets all hard constraints.

Commit to the Route Before Construction

The biggest route selection mistake is beginning construction and then making major course changes mid-project. Changes after construction starts waste all labor on the abandoned section and often produce awkward junctions and grades at the change point. Walk every option thoroughly, make the decision, document it, and then build without second-guessing.