I. INTRODUCTION
In this chapter, capacity and quality of service concepts for urban streets are
introduced. The term “urban streets,” as used in this manual, refers to urban arterials and
collectors, including those in downtown areas. Methodologies found in Chapter 15
(Urban Streets), Chapter 16 (Signalized Intersections), and Chapter 17 (Unsignalized
Intersections) can be used in conjunction with this chapter.
II. URBAN STREETS
In the hierarchy of street transportation facilities, urban streets (including arterials
and collectors) are ranked between local streets and multilane suburban and rural
highways. The difference is determined principally by street function, control conditions,
and the character and intensity of roadside development.
Arterial streets are roads that primarily serve longer through trips. However,
providing access to abutting commercial and residential land uses is also an important
function of arterials. Collector streets provide both land access and traffic circulation
within residential, commercial, and industrial areas. Their access function is more
important than that of arterials, and unlike arterials their operation is not always
dominated by traffic signals.
Downtown streets are signalized facilities that often resemble arterials. They not
only move through traffic but also provide access to local businesses for passenger cars,
transit buses, and trucks. Turning movements at downtown intersections are often greater
than 20 percent of total traffic volume because downtown flow typically involves a
substantial amount of circulatory traffic.
Pedestrian conflicts and lane obstructions created by stopping or standing taxicabs,
buses, trucks, and parking vehicles that cause turbulence in the traffic flow are typical of
downtown streets. Downtown street function may change with the time of day; some
downtown streets are converted to arterial-type operation during peak traffic hours.
Multilane suburban and rural highways differ from urban streets in the following
ways: roadside development is not as intense, density of traffic access points is not as
high, and signalized intersections are more than 3.0 km apart. These conditions result in
a smaller number of traffic conflicts, smoother flow, and dissipation of the platoon
structure associated with traffic flow on an arterial or collector with traffic signals.
The urban streets methodology described in this chapter and in Chapter 15 can be
used to assess the mobility function of the urban street. The degree of mobility provided
is assessed in terms of travel speed for the through-traffic stream. A street’s access
function is not assessed by this methodology. The level of access provided by a street
should also be considered in evaluating its performance, especially if the street is
intended to provide such access. Oftentimes, factors that favor mobility reflect minimal
levels of access and vice versa.
The functional classification of an urban street is the type of traffic service the street Functional class defined
provides. Within the functional classification, the arterial is further classified by its
design category. Illustrations 10-1 through 10-4 show typical examples of four design
categories that are described in the following sections.