2.2.4 Vertical Alignment
The design of the vertical alignment of a roadway also has a direct effect on the safety and
comfort of the driver. Steep grades can slow down large, heavy vehicles in the traffic
stream in the uphill direction and can adversely affect stopping ability in the downhill
direction. Grades that are flat or nearly flat over extended distances will slow down the rate
at which the pavement surface drains. Vertical curves provide a smooth change between
two tangent grades, but must be designed to provide adequate stopping sight distance.
Tangent Grades. The maximum percent grade for a given roadway is determined by its
functional classification, surrounding terrain, and design speed. Table 2.16 shows how
the maximum grade can vary under different circumstances. Note that relatively flat
grade limits are recommended for higher functional class roadways and at higher design
speeds, whereas steeper grade limits are permitted for local roads and at lower design
speeds.
Concerning minimum grades, flat and level grades may be used on uncurbed
roadways without objection, as long as the pavement is adequately crowned to drain the
surface laterally. The preferred minimum grade for curbed pavements is 0.5 percent,
but a grade of 0.3 percent may be used where there is a high-type pavement accurately
crowned and supported on firm subgrade.
Critical Length of Grade. Freedom and safety of movement on two-lane highways
are adversely affected by heavily loaded vehicles operating on upgrades of sufficient
lengths to result in speeds that could impede following vehicles. The term critical
length of grade is defined as the length of a particular upgrade which reduces the
operating speed of a truck with a weight-to-horsepower ratio of 200 lb/hp (0.122 kg/W)
to 10 mi/h (1.6 km/h) below the operating speed of the remaining traffic. Figure 2.11
provides the amount of speed reduction for these trucks given a range of percent
upgrades and length of grades. The entering speed is assumed to be 70 mi/h (113 km/h).
The curve representing a 10-mi/h (1.6-km/h) reduction is the design guideline to be
used in determining the critical length of grade.
2.2.4 Vertical AlignmentThe design of the vertical alignment of a roadway also has a direct effect on the safety andcomfort of the driver. Steep grades can slow down large, heavy vehicles in the trafficstream in the uphill direction and can adversely affect stopping ability in the downhilldirection. Grades that are flat or nearly flat over extended distances will slow down the rateat which the pavement surface drains. Vertical curves provide a smooth change betweentwo tangent grades, but must be designed to provide adequate stopping sight distance.Tangent Grades. The maximum percent grade for a given roadway is determined by itsfunctional classification, surrounding terrain, and design speed. Table 2.16 shows howthe maximum grade can vary under different circumstances. Note that relatively flatgrade limits are recommended for higher functional class roadways and at higher designspeeds, whereas steeper grade limits are permitted for local roads and at lower designspeeds.Concerning minimum grades, flat and level grades may be used on uncurbedroadways without objection, as long as the pavement is adequately crowned to drain thesurface laterally. The preferred minimum grade for curbed pavements is 0.5 percent,but a grade of 0.3 percent may be used where there is a high-type pavement accuratelycrowned and supported on firm subgrade.Critical Length of Grade. Freedom and safety of movement on two-lane highwaysare adversely affected by heavily loaded vehicles operating on upgrades of sufficientlengths to result in speeds that could impede following vehicles. The term criticallength of grade is defined as the length of a particular upgrade which reduces theoperating speed of a truck with a weight-to-horsepower ratio of 200 lb/hp (0.122 kg/W)to 10 mi/h (1.6 km/h) below the operating speed of the remaining traffic. Figure 2.11provides the amount of speed reduction for these trucks given a range of percentupgrades and length of grades. The entering speed is assumed to be 70 mi/h (113 km/h).The curve representing a 10-mi/h (1.6-km/h) reduction is the design guideline to beused in determining the critical length of grade.
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