Civil Engineering Reference
In-Depth Information
Chapter 12
The Impacts of Congestion on Roadway
Traf
c Productivity
12.1 Introduction
Road productivity can be de
c volume (vehicles or per-
sons per hour) of a roadway at a given point, or as the person-miles or vehicle-miles
per hour that can be moved on a roadway segment, or on an area-wide road network.
This chapter identi
ned as the throughput traf
c elements (throughput volume, capacity,
speed, and density) that determine road productivity. Through illustrative examples it
shows how productivity of freeways and arterial streets is impacted by congestion.
es the various traf
12.2 Fundamentals of Traffic Flow
Traf
flow theory provides an analytical means of evaluating capacity, congestion,
and productivity. It is useful in analyzing the effects of changes in traf
c
fl
c demand,
fl
flow, and speeds over time. For example, it can assist in answering questions such
as: what are the congestion and productivity impacts of a new land development
along a heavily traveled roadway?
12.2.1 Basic Relationships
Traf
c speed is a basic indicator of congestion. Speed depends upon (1) type of
roadway (freeway, expressway, arterial, or collector streets); (2) the roadway
geometry and controls (from a roadway that is fully accessible from adjacent land
use activities, to a roadway with full access control); (3) the nature, extent, and
duration of traf
c con
fl
icts/incidents that interfere with traf
c
fl
ow; and (4) the
