CAPACITY ANALYSIS AND LEVEL-OF-SERVICE intersections controlled by two-way STOP or YIELD
ANALYSIS ON AN UNSIGNALIZED signs).
INTERSECTION
A. CAPACITY ANALYSIS
The overall objective of the study was an
The computational steps involve identifying the
empirical comparison of procedures to estimate
non-prior to movements (all movements from
unsignalized intersection capacity and level of
the minor approaches and left turns from the
service. Before the empirical analysis was
major road) / determining the traffic streams
conducted, each major capacity analysis method
conflicting with each of the nonpriority
was reviewed in terms of theoretical structure,
movements, and then applying the appropriate
applicability to a broad range of traffic conditions,
critical gaps.
and computational procedure.
The maximum (or potential) capacity for each
An overview of the study methodology for movement is read from the graph of maximum
the empirical testing stated: First, each major capacity versus conflicting traffic stream.
component of the procedures (critical gaps, These capacities are then adjusted to account
capacity, and level of service) is analyzed for intersection congestion caused by the
individually. Based on the comparison of theoretical servicing of conflicting movements.
and computational structure as well as the The existing (or projected) traffic demand for
empirical results, each model is revised and again the movement is converted to passenger car
examined with respect to field data. The equivalents per hour (pch) to account for
methodology sought to isolate the prediction errors approach grade and traffic mix. This demand is
that were due to model structure from those that deducted from the calculated capacity to give
were due to differences in drivers and driving what is called the reserve capacity. Reserve
conditions, which were captured in the Swedish capacity determines what level of service (A
and German data. through F) is assigned to the movement and
describes the traffic delay that will be
The analysis of the two unsignalized
expected. If minor street delays are a function
intersection capacity procedures includes the
of critical gap and major street volume, reserve
following specific steps.
capacity should correlate fairly well with delay.
1. Comparison of the conceptual and This capacity model assumes that
computational structure of the methods; 1. Vehicles on the major road arrive at
2. Collection of data on gaps at four two-way random;
stop-sign controlled intersections and analysis of 2. Minor road vehicles do not force their way
these data to estimate the mean critical gaps for into the intersection and
the minor road movements. 3. Priority is given to traffic movements in
3. Comparison of the critical gaps obtained from the following sequence: main street through
field data with those in Circular 212, the and right turn vehicles, minor street right turns,
subcommittee report, and the Swedish manual. major street left turns, minor street vehicles
4. Determination of the capacities of minor road crossing the major street, and minor street left
approaches and the level of service to minor road turns.
vehicles at the studied intersections using two
B. LEVEL OF SERVICE
capacity methods followed by comparison of these
results with observed capacities and delays; and Level of service variables for freeways
5. Recommendation of revisions to the Circular (density), arterials (average speed), and
212 and Swedish procedures that would make the signalized intersections (stopped delay) can be
procedures more closely reflect U.S. drivers and measured in the field and compared with
driving conditions. model predictions. Reserve capacity is a
function of model estimates and therefore
*Transportation Research Circular 212 --The cannot be independently measured.
procedure in Circular 212 (1) is a method for
computing the capacity and level-of service of
priority-type non-signalized intersections (i.e., those
