Module 5 : Uninterrupted Flow
Lecture 24 : Freeway Operations
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Basic features of freeway

Freeway provides uninterrupted traffic flow on a freeway. Traffic on freeway is free-flowing. All cross-traffic (and left-turning traffic) is relegated to overpasses or underpasses, so that there are no traffic conflicts on the main line of the highway which must be regulated by traffic lights, stop signs, or other traffic control devices. Specific features are:
  1. There are no signalized or stop-controlled at-grade intersections.
  2. Direct access to and from adjacent property is not permitted.
  3. Access to and from the freeway is limited to ramp locations.
  4. Opposing directions of flow are continuously separated by a raised barrier, an at-grade median, or a continuous raised median.
  5. The advantage of grade-separated interchanges is that freeway drivers can almost always maintain their speed at junctions since they do not need to yield to vehicles crossing perpendicular to mainline traffic.
A freeway is composed of following three components
  1. Basic freeway segment
  2. Ramp junction
  3. Weaving areas

Basic freeway segment

Basic freeway are that part of segment of freeway which are outside of the influence area of ramps or weaving areas of freeway.
Figure 1: Basic freeway segment
\begin{figure} \centerline{\epsfig{file=qfFreeBasicSegment.eps,width=8cm}} \end{figure}
We can see in Fig.1 that a basic freeway segment is independent of the ramps and weaving areas and the flow in such section occurs smoothly at the much larger rates. Merging and diverging of traffic occurs where on-or-off ramps join the basic freeway segment. Weaving occurs when vehicles cross each other's path while travelling on freeway lanes. The exact point at which basic freeway segment begins or ends- that is, where the influence of weaving areas and ramp junctions has dissipated- depends on local conditions, particularly the level of service operating at the time. If traffic flow is light, the influence may be negligible, whereas under congested conditions, queues may be extensive.

Base condition of basic freeway segment

The base conditions under which the full capacity of a basic freeway segment is achieved are good weather, good visibility, and no incidents or accidents. For the analysis procedures in this chapter, these base conditions are assumed to exist. A set of base conditions for basic freeway segments has been established. These conditions serve as a starting point for the
  1. Lane widths of 3.6 m,
  2. Clearance of 1.8 m between the edge of the travel lanes and the nearest obstructions or objects at the roadside and in the median,
  3. Free-flow speed of 120 km/h for freeways,
  4. Only passenger cars in the traffic stream (no heavy vehicles),
  5. Level terrain,
  6. No no-passing zones on two-lane highways, and
  7. No impediments to through traffic due to traffic control or turning vehicles.
Base conditions for intersection approaches include the following:
  1. Lane widths of 3.6 m,
  2. Level grade,
  3. No curb parking on the approaches,
  4. Only passenger cars in the traffic stream,
  5. No local transit buses stopping in the travel lanes,
  6. Intersection located in a non central business district area, and
  7. No pedestrians