The three principal values controlling design of roads and streets
are average daily traffic volume (ADT), design speed, and design vehicle.
These values form the basis for the selection of the geometric elements
that are required to accommodate the anticipated traffic at a desired
quality of service.
A. Average daily traffic volume (ADT).
(1) Average daily traffic volume will be the design control as shown in Article
VII, §
278-72, Functional classification of roads, of the Charles County Subdivision Regulations. In cases of lower classification of streets, trip generation rates may be used in lieu of average daily traffic.
B. Design speed.
(1) Design speed is the maximum safe speed that can be maintained over
a given section of road when the traffic volume is so low that the
geometrics of the roadway control speed. All elements should be in
balance consistent with a specified design speed. Stopping sight distance,
horizontal and vertical alignment, and superelevation are among the
roadway elements which are controlled by design speed. The minimum
design values for the various elements required for a given design
speed should be used only where controls such as topography and property
damages dictate their use. On local and minor collector residential
streets it is desirable to keep speeds low, and care must be exercised
that the design does not encourage high speeds without sacrificing
any of the design standards included herein. Speed control measures
shall be incorporated into the design of all residential local and
minor collector roads longer than 1,500 feet which are located within
the development district.
[Amended 5-12-2015 by
Bill No. 2015-03]
(2) Unless otherwise approved by the County, the minimum design speeds listed in Tables 2.01.01 and 2.01.02 (
Appendix A) shall be used for the design of roads. For existing County roads whose classification is unknown
the design speed shall be the posted speed plus 10 mph.
C. Design vehicle.
(1) The design vehicle is the motor vehicle whose characteristics determine
such geometric elements as vertical clearance and turning radii. Vehicles
have been divided into six classes and a typical design vehicle has
been developed for each. The respective design vehicles have dimensions
and a minimum turning radius larger than those of almost all vehicles
in the respective classes.
(2) The six design vehicle designations are as follows:
(d)
WB-40 (Semitrailer combination, intermediate).
(e)
WB-50 (Semitrailer combination, large).
(f)
WB-60 (Semitrailer - full trailer combination).
(3) Dimensions and turning characteristics of each design vehicle are
contained in Reference 1.
(4) Though a road must be designed to accommodate the largest vehicle
likely to use it, the selection of too large a design vehicle can
have an adverse effect, such as longer crosswalks at the intersections.
The design vehicle must be selected considering the street classification
and adjacent land uses. Its selection is subject to review and approval
by the County.
(5) In no case shall a public road, private road or commercial/industrial
entrance be so designed that it cannot accommodate a WB-40 vehicle
and a fire truck with an inside turning radius of 35.5 feet and an
outside turning radius of 46 feet and an overhang of 2.5 feet.
All paving sections shall be in accordance with Table 2.07 (
Appendix A) and the Standard Details.
Roundabouts shall be designed in accordance with the most recent
Maryland State Highway Administration Roundabout Design Guidelines.
Curb openings shall be constructed in accordance with the MSHA
Standards.
Urban road standards shall be applied within the Urban Road Standards District. A map of this district is shown in
Appendix G. Design criteria are contained in Table 2.01.02 (
Appendix A).