Understanding how roadway grade influences vehicle speeds is a prerequisite for designing vertical alignments that facilitate uniform operations across vehicle classes. Grade equals the change in roadway elevation divided by horizontal distance. For example, if elevation increases 4 ft. over a horizontal distance of 100 ft., the roadway grade equals 4% — grade is usually expressed as a percentage. Sight distances, vehicle stopping distances, and vehicle speeds are all influenced by roadway grade.

How grade impacts vehicle speeds varies by vehicle class. Passenger cars, for instance, can ascend steep grades (4% – 5%) without significant speed reductions relative to speeds maintained on level roadways. Conversely, heavy trucks encounter greater operational challenges navigating steep grades. Compared to level roadways, truck speeds are up to 5% higher on downgrades and at least 7% lower on upgrades. Also, trucks require substantially longer stopping distances on steep upgrades due to their weight, momentum, and braking limitations. When speeds for large and small vehicles differ, roadway safety and operational efficiency worsen (e.g., greater speed variability increases crash risk). AASHTO’s three-tiered classification system characterizes how road topography affects vehicle operations (Table 1).

Designers select a maximum roadway grade based on several factors, including terrain type (see Section 2.1). Kentucky has an enormously diverse topography, ranging from bottomlands in the west and rolling hills in central areas of the state, to the Appalachian Mountains in the east. Topographic variability is most pronounced in eastern portions of the state. For example, Harlan County has the most dramatic relief, with elevations bottoming out in the Cumberland River lowlands at 870 ft. and peaking at Black Mountain (4,145 ft.) — a difference of nearly 3,300 ft. To the west, topographic variability is muted. Fulton County, which is in the extreme southwestern portion of the state, borders the Mississippi River and elevations range from 257- 510 ft.

2.1 Maximum and Minimum Grades

With a few exceptions, maximum grade is the highest grade that should be incorporated into a roadway’s design. Selection of a maximum grade is based on (1) context and functional classification, (2) terrain/land use, and (3) design speed (Table 2). To the table’s organization, Freeways and Interstates are listed under context classifications, despite not being formal context classifications.

To determine an appropriate maximum grade using Table 2, identify the context classification, functional classification and terrain/land use (in that order). Next, locate the facility’s design speed at the top of the table. Then, find the cell beneath the design speed in the row which corresponds to the road type selected. For example, a rural collector in mountainous terrain with a 45 mph design speed has a maximum grade of 10%. Generally, maximum grades decrease as design speeds increase.

Remember that maximum grade is not the preferred grade. The Green Book recommends infrequent use of maximum grades and keeping grades as moderate as practical. Grade selection has a direct influence on earthwork quantities.  Depending on the terrain, a steeper grade may reduce or increase excavation and embankment volumes based on how closely the vertical alignment follows existing ground. Designing the grade line to better track —or — “chase” — the natural terrain, is a key design tool for balancing earthwork and minimizing extreme cuts, fills, and material hauling. This approach cuts down hauling costs and reduces the amount of material that must be brought into or removed from the project. Designers also need to be attentive to the project context and adjacent roadway segments that lie beyond the project limits to ensure consistent user experiences and to support uniform operations.

Grades steeper than the maximum values listed in Table 2 can be an option if a milder grade would result in extremely high construction costs. There are two other situations where grades steeper than the maximum values may be used. For grades < 500 ft. and one-way downgrades, the grade may be 1% steeper than the maximum value. On low-volume rural highways, it may be 2% steeper. Designers also need to be careful when increasing grade in rural areas because doing so may introduce the need for truck climbing lanes (see the HKP article Truck Climbing Lanes for details).

In most cases a minimum grade of 0.5% is sufficient for adequate drainage, however, a grade of 0.3% may be considered if there is adequately crowned pavement on firm subgrade. On very level terrain, it may be necessary to make small, gradual vertical adjustments to the profile to avoid flat spots.  This is sometimes referred to as “rolling” the grade. If an uncurbed roadway has crowned pavement with a cross slope (e.g., 2%) steep enough to support proper surface drainage, a level grade may be used. However, on curbed roadways a minimum grade must be established to provide adequate drainage.

Where possible, do not use grades on intersections. However, if doing so is unavoidable, the preferred practice is to minimize the gradient at the intersection as this benefits turning vehicles and lowers crash risk.

2.2 Critical Length of Grade

Maximum grade is not the only grade-related design consideration. Another important factor is critical length of grade, which is the maximum length of a specific upgrade on which a loaded truck can operate without an unreasonable reduction in speed — generally 10 mph (AASHTO Green Book, p. 3-130 – 3-131). Speed reductions greater than 10 mph introduce greater variance in vehicle operating speeds, which increases crash risk. When the length of grade is less than the critical value, acceptable operations within the preferred range of speeds are possible. Figure 3-21 in AASHTO’s Green Book can be used to determine the critical length of grade for different grades and levels of speed reduction.

If possible, avoid using grades that require trucks to slow down to the point where motorists following behind become frustrated and attempt unsafe passing maneuvers. On roads with high traffic volumes and a large number of heavy trucks, using steep grades on downhill sections can reduce traffic capacity and increase crash frequency.  When large speed differentials are expected, evaluate whether design features that provide regular passing opportunities are appropriate. These include auxiliary lanes (including truck climbing lanes, discussed above), periodic passing lanes, or 2+1 roadways (see the HKP article KYTC’s 2+1 Roadway Design Guidance for more details).

KYTC Highway Design Manual. HD-700. 

KYTC Standard Drawing RPM-110-07. 

American Association of State Highway and Transportation Officials. (2016). A policy on design standards—Interstate system (6th ed.). Washington, DC. https://store.transportation.org/item/collectiondetail/180

American Association of State Highway and Transportation Officials. (2018). A Policy on the Geometric Design of Highways and Streets, 7th Edition. American Association of State Highway and Transportation Officials, Washington, D.C. https://store.transportation.org/item/collectiondetail/180  

FHWA. (2016). Highway Performance Monitoring System Field Manual. FHWA. https://www.fhwa.dot.gov/policyinformation/hpms/fieldmanual/hpms_field_manual_dec2016.pdf  

Truck Climbing Lanes

KYTC’s 2+1 Roadway Design Guidance