TABLE 1 AASHTO (1990) Maximum Grades for Urban and Rural Freeways

DESIGN SPEED (mph) = 50 DESIGN SPEED (mph) = 60 DESIGN SPEED (mph) = 70

TERRAIN GRADES (percentage)
Level 4 3 3
Rolling 5 4 4
Mountainous 6 6 5
In addition to the main consideration of cars' The separate selection of the maximum grade and the
performance on grades, the maximum slope is also maximum length and the lack of a theoretical basis for the
determined by economic considerations. Urban or suburban former leads inconsistencies between standards.
to For
freeways, with large volumes of traffic, may justify higher example, it is possible to find that some design standards
construction costs than two-lane rural highways with very (for example AASHTO, 1990) at 80 kph (50 mph), allow
low volumes. The latter may require higher maximum use of 9% slope at a length of less than 150 meters (500 ft.).
slopes to make the construction more economical, by Another standard (for example the South African TRH 17,
reducing earth-works, reducing hauling, etc. 1984), will not even moderate slope of about
allow a more
Maximum grade in itself is only a partial design 8% at the same Additionally, it may be found
design speed.
criterion. achieve a quality balanced design, it is
To (for example AASHTO, 1990) that a moderate slope of 3%
necessary to consider the length of the grade. Most is unlimited in length (for passenger cars and recreational
standards do not explicitly limit the length of slopes, but vehicles) while the same slope is restricted to 400 meters
suggest that it is desirable to limit the length of sections with (1300 ft.) in TRH 17 (1984); also in the Scottish standards
maximum slopes. AASHTO (1990) proposes limiting the (1968), the same 3% slope is limited to 500 meters (1600
maximum length to that which will not exceed the "critical ft.).
length of grade." The critical length is that which will cause All this leads to inconsistencies among standards and the
a typical loaded truck (300 pound/horsepower) to "operate lack of a common guideline for highway engineering
without an unreasonable reduction in speed" (AASHTO, projects.
1990). 10 mph is recommended, the reason
A reduction of
PROPOSED CONCEPT
being the significant increase in accident involvement rate
at higher speed reductions.
Because of the inconsistencies and limitations of the existing
LIMITATIONS OF CURRENT DESIGN APPROACH methods for determining the maximum grade and its
maximum length based on the design speed, a new concept
A review of the literature shows that the control of the is proposed. This concept is based on the following
vertical alignment according to most design standards principles:
should be conducted in a two-step process: First is an combining both grade magnitude and length to a single
arbitrary selection ofthe maximum grade, based on a certain design element (i.e., joining the two parameters into
design speed and terrain type. In most design standards, the one);
determination of the maximum grade is generally determining the combined design control according to
recommended without a theoretical basis. The the performance characteristics of both cars and trucks;
recommendation is based on general engineering judgment adhering to stringent safety considerations, in order to be
representing local conditions; however, it is not based on consistent on grades, as well as on level sections, along
analytical calculations of the performance of trucks on the entire alignment.
grades. Second, an analytical selection is made of the The most important practical implication of these
maximum length allowed for a given grade. The limitation principles is that it will be possible to use any grade when
of length is proposed only for the maximum allowable designing a vertical alignment for any design speed. While
grades, because steeper grades are not permitted even for steep grades could be used for short distances, moderate
small lengths. slopes could be designed for longer lengths, and very low
grades could be unlimited in length.
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