Researchers developed a safety performance function(SPF), which estimates the probability of occurrence of a crash for a given segment of roadway. Using data on over 100 segments ofroadway, they fit the model Upper E left parenthesis y right parenthesis equals beta 0 plus beta 1 x 1 plus beta 2 x 2

E(y)=0+1x1+2x2, where y equals number

y=number of crashes per threeyears, x 1 equals

x1=roadway length(miles), and x 2 equals

x2=average annual daily traffic(number ofvehicles) equals

=AADT.

Interstate Highways

Variable

Parameter Estimate

Standard Error

t-value

Intercept

1.87537

1.87537

0.41183

0.41183

3.64

3.64

Length left parenthesis x 1 right parenthesis

x1

0.11567

0.11567

0.02583

0.02583

4.75

4.75

AADT left parenthesis x 2 right parenthesis

x2

0.00011

0.00011

0.00003

0.00003

5.55

5.55

Non-Interstate Highways

Variable

Parameter Estimate

Standard Error

t-value

Intercept

1.17156

1.17156

0.35305

0.35305

3.24116

3.24116

Length left parenthesis x 1 right parenthesis

x1

0.59185

0.59185

0.02622

0.02622

3.82897

3.82897

AADT left parenthesis x 2 right parenthesis

x2

0.00046

0.00046

0.00016

0.00016

6.70092

6.70092