Reverse-In Parking in Hawaii and California

Introduction

One of my mainland-born friends is annoyed that Hawaii drivers like to reverse their cars into parking stalls much more than the mainland. My wife, who is also from the mainland, agreed with him enthusiastically. Whether this is appropriate parking behavior or not, I had never noticed it before, so I figured it was something I could test with some observations. I had a trip coming up to the mainland, and I was able to collect some data to test whether the Hawaii drivers were really so different than their California counterparts.

Methods

I took photographs of parked vehicles in the parking lot of the 99 Ranch mall in Anaheim, California. This is a mixed use mall with a variety of restaurants and small retail and service businesses. The Hawaii comparison photographs were taken in Koko Marina and the Ward area parking structure, which also have a variety of restaurants and retail businesses.

I recorded how many vehicles were facing in and how many were facing out in each photograph.

I included all vehicles that were parked in stalls that were perpendicular to the traffic direction. I excluded non-passenger vehicles such as semi-trucks or commercial vans.

I estimated beforehand that the proportion of reversed-in cars in Hawaii and California would be about 30% and 15% respectively. With a Type 1 error rate of 5% and a Type 2 error rate of 20%, the sample size required was 118 cars per group. (https://select-statistics.co.uk/calculators/sample-size-calculator-two-proportions/)

I analyzed the data using Fisher’s exact test, with a two sided probability. Epidemiologic risk based estimates were calculated using the epiR package of R.

Results

Representative photos for the parking lots are in Figure 1.

I obtained data on 172 vehicles in California and 197 vehicles in Hawaii. In California, 18/172 (10.4%, 95% confidence interval 6.6-16.0%) vehicles were reversed in. In Hawaii, 68/197 (34.5%, 95% CI 28.2-41.4%) vehicles were reversed in.

Vehicles in Hawaii were 3.3 (2.0-5.3) times more likely to be reversed in than in California. The two-sided p-value was less than 0.0001.

Here’s the code for the analysis.

library(dplyr)

## 
## Attaching package: 'dplyr'

## The following objects are masked from 'package:stats':
## 
##     filter, lag

## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union

library(tidyr)
library(epiR)

## Loading required package: survival

## Package epiR 0.9-82 is loaded

## Type help(epi.about) for summary information

## 

fileURL <- "https://mching.github.io/datasets/parking_data.csv"
download.file(fileURL, destfile = "../datasets/parking_data.csv", method = "curl")
dat <- read.csv("../datasets/parking_data.csv")
dat <- tbl_df(dat)
dat <- dat %>% gather(Direction, n, Reverse_in:Forward_in)
dat <- dat[rep(1:nrow(dat), dat$n), 1:3]

dat$State <- factor(dat$State, levels = c("Hawaii", "California"))
dat$Direction <- factor(dat$Direction, levels = c("Reverse_in", "Forward_in"))

table1 <- table(dat$State, dat$Direction)
table1

##             
##              Reverse_in Forward_in
##   Hawaii             68        129
##   California         18        154

fisher.test(table1)

## 
##  Fisher's Exact Test for Count Data
## 
## data:  table1
## p-value = 3.357e-08
## alternative hypothesis: true odds ratio is not equal to 1
## 95 percent confidence interval:
##  2.488736 8.458416
## sample estimates:
## odds ratio 
##   4.492008

epi.2by2(table1, method = "cross.sectional")

##              Outcome +    Outcome -      Total        Prevalence *
## Exposed +           68          129        197                34.5
## Exposed -           18          154        172                10.5
## Total               86          283        369                23.3
##                  Odds
## Exposed +       0.527
## Exposed -       0.117
## Total           0.304
## 
## Point estimates and 95 % CIs:
## -------------------------------------------------------------------
## Prevalence ratio                             3.30 (2.05, 5.32)
## Odds ratio                                   4.51 (2.55, 7.97)
## Attrib prevalence *                          24.05 (15.99, 32.12)
## Attrib prevalence in population *            12.84 (6.55, 19.13)
## Attrib fraction in exposed (%)              69.68 (51.12, 81.19)
## Attrib fraction in population (%)           55.10 (34.26, 69.33)
## -------------------------------------------------------------------
##  X2 test statistic: 29.721 p-value: < 0.001
##  Wald confidence limits
##  * Outcomes per 100 population units

Discussion

It wasn’t even close. The observed data support the hypothesis that Hawaii drivers like to reverse into stalls much more than California drivers. The effect size was dramatic, with more than 3 times more cars reversed in Hawaii than California.

The reason for this behavior is unknown but there is much online speculation about it. The Hawaii Driver Manual, page 64 recommends reversing in so that drivers can enter traffic in a forward direction. However, this appears to be mainly about entering a roadway rather than a parking lot lane.

More on the controversy from the Huffington Post And Slate And NPR (a study done in China versus the USA)

Limitations of the study include the convenience sample. Clustering was not taken into account in the analysis. In addition, there may be other factors that affect the parking in specific areas such as the neighborhoods, retail mix, time of day, percentage of stalls filled, etc. While I tried to match the types of businesses, most of the Hawaii cars came from a nearly full, enclosed parking garage with one-way lanes, while the California lot was virtually empty.

Like this current study, any future work would be frivolous and gratuitous, but possibilities include collecting data from other parts of the mainland, limiting observations to cars parked up against a barrier (like a sidewalk or building), and matching more closely the stores and neighborhoods (i.e., comparing Safeway parking lots in suburban locations).

Conclusion

In Hawaii drivers reverse into parking stalls at a higher rate than in California. No comment is made on the appropriateness of reversing in, but I, for one, like doing it.