Question

Yellow light: When the light turns yellow, should you stop or go through it? A recent study of driver behavior defined the "indecision zone" as the period when

a vehicle is between 2.5 and 5.5 seconds away from an intersection. At the first intersection studied, 127 vehicles were observed to encounter a yellow light in
the indecision zone, and 24 of them ran the red light. At the second intersection, 164 vehicles entered the intersection in the indecision zone, and 29 ran the red
light. Let p denote the proportion of red light runners at the first intersection. Can you conclude that the proportion of red light runners differs between the two
intersections? Use the a=0.01 level of significance.

Answer 1

Explanation:
To test the null hypothesis that the proportion of red light runners is the same at both intersections, we can use a two-sample z-test for proportions.

Let p1 be the proportion of red light runners at the first intersection, and p2 be the proportion of red light runners at the second intersection. We want to test the null hypothesis:

H0: p1 = p2

against the alternative hypothesis:

Ha: p1 ≠ p2

We can use the following formula to calculate the test statistic:

z = (p1 - p2) / sqrt(p_hat * (1 - p_hat) * (1/n1 + 1/n2))

where p_hat = (x1 + x2) / (n1 + n2) is the pooled sample proportion, and x1 and x2 are the number of red light runners at the two intersections, and n1 and n2 are the sample sizes.

For the first intersection, we have x1 = 24 and n1 = 127. For the second intersection, we have x2 = 29 and n2 = 164.

The pooled sample proportion is:

p_hat = (x1 + x2) / (n1 + n2) = (24 + 29) / (127 + 164) = 0.167

The test statistic is:

z = (p1 - p2) / sqrt(p_hat * (1 - p_hat) * (1/n1 + 1/n2)) = (0.189 - 0.177) / sqrt(0.167 * (1 - 0.167) * (1/127 + 1/164)) = 0.99

Using a standard normal distribution table, we can find that the probability of getting a z-value of 0.99 or greater is 0.160. Since this is greater than the significance level of 0.01, we fail to reject the null hypothesis.

Therefore, we cannot conclude that the proportion of red light runners differs between the two intersections.