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The profile line is a reference line by which the elevation of the pavement and other features of the highway are established. It is controlled mainly by topography， type of highway， horizontal alignment， safety， sight distance， construction costs， cultural development， drainage and pleasing appearance. The performance of heavy vehicles on a grade must also be considered. All portions of the profile line must meet sight distance requirements for the design speed of the road.
In flat terrain， the elevation of the profile line is often controlled by drainage considerations. In rolling terrain， some undulation in the profile line is often advantageous， both from the standpoint of truck operation and construction economy. But， this should be done with appearance in mind； for example， a profile on tangent alignment exhibiting a series of humps visible for some distance ahead should be avoided whenever possible. In rolling terrain， however， the profile usually is closely dependent upon physical controls.
Separate or independent profile lines are appropriate in some cases for freeways and divided arterial highways.
They are not normally considered appropriate where medians are less than 30 feet. Exceptions to this may be minor differences between opposing grade lines in special situations.
In addition， appreciable grade differentials between roadbeds should be avoided in the vicinity of at-grade intersections. For traffic entering from the crossroad， confusion and wrong-way movements could result if the pavement of the far roadway is obscured due to an excessive differential.
The minimum grade rate for freeways and land service highways with a curbed or bermed section is 0.3 percent. On highways with an umbrella section， grades flatter than 0.3 percent may be used where the shoulder width is 8 feet or greater and the shoulder cross slope is 4 percent or greater.
For maximum grades for urban and rural land service highways and freeways， see Tab. 1.
Tab. 1 Maximum grades（％）
Properly designed vertical curves should provide adequate sight distance， safety， comfortable driving， good drainage， and pleasing appearance. On new alignments or major reconstruction projects on existing highways， the designer should， where practical， provide the desirable vertical curve lengths. The use of minimum vertical curve lengths should be limited to existing highways and those locations where the desirable values or values greater than the minimum would involve significant social， environmental or economic impacts.
Except on level terrain， often it is not economically feasible to design a profile that will allow uniform operating speeds for all classes of vehicles. Sometimes， a long sustained gradient is unavoidable.
From a truck operation standpoint， a profile with sections of maximum gradient broken by length of flatter grade is preferable to a long sustained grade only slightly below the maximum allowable. It is considered good practice to use the steeper rates at the bottom of the grade， thus developing slack for lighter gradient at the top or elsewhere on the grade.
A proper balance between curvature and grades should be sought. When possible， vertical curves should be superimposed on horizontal curves. This reduces the number of sight distance restrictions on the project， makes changes in profile less apparent， particularly in rolling terrain，and results in a pleasing appearance. For safety reasons， the horizontal curve should lead the vertical curve. On the other hand， where the change in horizontal alignment at a grade summit is slight， it safely may be concealed by making the vertical curve overlay the horizontal curve.
When vertical and horizontal curves are thus superimposed， the superelevation may cause distortion in the outer pavement edges. Profiles of the pavement edge should be plotted and smooth curves introduced to remove any irregularities.
A sharp horizontal curve should not be introduced at or near a pronounced summit or grade sag.This presents a distorted appearance and is particularly hazardous at night.
A climbing lane is an auxiliary lane introduced at the beginning of a sustained positive grade for the diversion of slow traffic.
Generally， climbing lanes will be provided when the conditions in Text B are satisfied. These conditions could be waived if slower moving truck traffic was the major contributing factor causing a high accident rate and could be corrected by addition of a climbing lane.
elevation n. 高程
level terrain. 平原
rolling terrain. 微丘
mountainous terrain. 山区
climbing lane 爬坡车道