Safety Evaluation of Intersection Conflict Warning Systems

Chapter 6. Development of Safety Performance Functions

This chapter presents the SPFs developed for each State, which are subsequently used in the EB methodology.⁽¹³⁾ Generalized linear modeling was used to estimate model coefficients assuming a negative binomial error distribution, which was consistent with the state of research in developing these models. In specifying a negative binomial error structure, the overdispersion parameter, k, was estimated iteratively from the model and the data. For a given dataset, smaller values of k indicated relatively better models.

SPFs were calibrated separately for intersections with two-lane and multilane major routes for Minnesota, Missouri, and North Carolina using the corresponding reference sites from each State. The SPFs developed are presented by State in the following sections.

The form of the SPFs for all States is given in figure 7.

Figure 7. Equation. SPF model form for all States.

Where:

TotalEntering = Total entering volume (major route AADT + minor route AADT).
characteristic = Intersection characteristics included in SPF, defined by associated estimated parameters c through s.

The following definitions were used for all States for parameters with intersection characteristics:

c = Proportion of total entering volume from minor approach.
d = Presence of flashers on the minor route approaches.
e = Presence of flashers on the major route approaches.
f = Number of major route left-turn lanes.
g = Number of minor route left-turn lanes.
h = Number of major route right-turn lanes.
i = Number of minor route right-turn lanes.
j = Advance intersection warning sign on major route approaches.
l = Posted speed limit on major route.
m = Presence of lighting at intersection.
n = Advance STOP AHEAD warning sign on minor route approaches.
o = Intersection angle.
p = Presence of channelization on minor route approaches.
q = Presence of dual stop signs on the minor route approaches.
r = Median width on major route.
s = Posted speed limit on minor route.
t = Presence of a median on major route.

In addition, the following parameter was provided for each SPF:

k = Overdispersion parameter of the model.

Minnesota Safety Performance Functions

The SPFs for two-lane at two-lane intersections are provided in table 12, and the SPFs for four‑lane at two-lane intersections are provided in table 13.

— Indicates not applicable.

— Indicates not applicable.

Missouri Safety Performance Functions

The SPFs for two-lane at two-lane intersections are provided in table 14, and the SPFs for four‑lane at two-lane intersections are provided in table 15.

— Indicates not applicable.

— Indicates not applicable.

North Carolina Safety Performance Functions

The SPFs for two-lane at two-lane intersections are provided in table 16, and the SPFs for four‑lane at two-lane intersections are provided in table 17.

— Indicates not applicable.

— Indicates not applicable.