SAFETY BELT SURVEY DESIGN IN ILLINOIS Mehdi Nassirpour, Ph.D. Illinois Department of Transportation.

SAFETY BELT SURVEY DESIGN IN ILLINOIS

Mehdi Nassirpour, Ph.D.Illinois Department of Transportation

OBJECTIVE OF THE SURVEY

•To design a multi-stage cluster random sample in order to estimate the statewide safety belt usage rate in Illinois.

Division of Traffic Safety at IDOT

Census Tracts Census Tracts Census TractsCensus Tracts

Road TypeRoad TypeRoad Type

HVR HVR LVRHVR LVR HVR LVRLVR

City of Chicago77 Communities

Cook CNo Chicago

30 Townships

Other Counties101 Counties

Urban Rural

Road Type

SAMPLE DESIGN SUMMARY IN ILLINOIS

Stage 1

Stage 2

Stage 3

ALLOCATION OF SAMPLE SITES

Total Sample (258 Sites

12 Counties (166 Sites)

Cook County (92 Sites)

Cook CountyNo Chicago (41 Sites)

City of Chicago (51 Sites)

STAGE 1 - PSU SELECTION

• Sampling Units - 20 PSU– 5 Community Areas in the City of Chicago– 3Townships in Cook County– 11 Counties (DuPage County was selected twice)– 4 out of 11 counties were rural counties. – 47 counties were eliminated due to a small number of

VMTs and population.– 54 counties contains about 90 percent of VMT and

population• Selection - Probability Proportional to Vehicle Miles of

Travel and Population.• Selection is systematic (1/Ns)

– After Random Start

SAFETY BELT LOCATIONS IN IL

MCHENRY COUNTYLOCATIONS (6)

LAKE COUNTYLOCATIONS (31)

KANE COUNTYLOCATIONS (14)

WILL COUNTYLOCATIONS (18)

CHICAGO LOCATIONS (49)

COOK COUNTYLOCATIONS (40)

DUPAGE COUNTYLOCATIONS (42)

CHAMPAIGN COUNTYLOCATIONS (9)

ROCK ISLANDCO. LOCATIONS (7)

PEORIA COUNTYLOCATIONS (9)

MONTGOMERGY COUNTYLOCATIONS (2)

ST. CLAIR COUNTYLOCATIONS (14)


STAGE 2 - CENSUS TRACT SELECTION

• Census Tracts – A total of 96 census tracts (10-12

percent of total tracts) within those selected counties were selected randomly.

• Selection - Probability Proportional to Population.

• Selection is systematic (1/Ns).– After Random Start


STAGE 3 - ROAD TYPE SELECTION

• Road Type – All roads (Interstates, freeways, US/IL

highways, and residential streets) within selected tracts were identified using tract maps.

– All Interstates and IL/US highways within these tracts were selected with certainty and other roads were selected randomly.


PROCEDURE USED TO ASSIGN OBSERVATION SITES TO OBSERVATION TIME PERIOD

• Selected roads randomly assigned to time of day and day of week.

• To minimize travel time and distance required to conduct the observations, the sampled sites were grouped into 29 geographic clusters, with each cluster containing about between 6 and 12 road sites.

• Each cluster is randomly (without replacement) assigned to a day of the week. Each day was divided into eight one-hour time periods between 7:00am to 6:30pm when the light was adequate for observation.


Weighting Procedures

Wctr = 1/Pctr

Pctr = Pc . Pt/c . Pr/ct

c identifies a sample of countyt identifies a sample of tractr identifies a sample of road sitePc probability of selecting county cPt/c probability of selecting tract t within

selected county cPr/ct probability of selecting road r, conditional

to the tract and county


FORMULA FOR ESTIMATING SAFETY BELT USAGE RATE

ctr

ctr

YctrWctr

XctrWctrR

.

.

R = Safety belt usage rateWctr = weight associated with road r in tract t in county c;Xctr = number of safety-belted front seat occupants of passenger cars and pick-up trucks observed in county c, tract t, and road site r; andYctr = number of front seat occupants observed in county c, tract t, and road site r.


FORMULA FOR ESTIMATING SAFETY BELT USAGE RATE

.

Octr

LctrFctr

FctrWctrctrW

YctrctrW

XctrctrWR

CTR

CTR

.'

.'

.'Fctr = adjustment for lanes not observed in road site r, tract t in county c;

Lctr = total number of lanes in road site r, tract t in county c; and

Octr = total number of observed lanes in road site r within tract t in county c

VARIANCE ESTIMATION

.

Estimated safety belt usage rate is subject to sampling error since a small number of front seat occupants were observed. These sampling errors are expressed as coefficients of variation (relative error), that are the ratios of the relative standard errors of estimates to the estimates themselves.


VARIANCE ESTIMATION PACKAGE

.

PC CARP from the Iowa state University was used to estimate the coefficient of variation for a ratio from a stratified cluster sample.

PC CARP is a menu driven program and is designed to construct estimates and standard errors for totals, means, ratios, differences of ratios, and entities in two-way tables for multi-stage stratified sample. The basic input for the program is the stratum identification, cluster identification, weight, and data vector for a set of observations.


VARIANCE ESTIMATION FORMULA

.

(Yijkl, Xijkl, Yijk2, Xijk2, ...., Yijkp, Xijkp)

where i= 1, 2, ....l j = 1, 2, ...,nj; k = 1, 2,...., mij; i is the stratum identification, j is the cluster identification, k is the element-within-cluster identification.

The above vector is used in the analysis and estimator is

.....3,2,1,.

.

1 1 1

1 1 1 psXijksWij

YijksWij

Xs

YsRs L

i

nij

j

mij

k

L

i

nij

j

mij

k

95 Percent ConfidenceInterval

EstimatedUsage Rate

Coefficientof Variance

StandardError

Lower Limit%

Upper Limit%

Statewide (258) 76.2 4 3 70 82

RegionsCity of Chicago (46) 69.1 8 5 59 79Cook County (40) 70.9 5 4 63 79Other Counties (172) 79 5 3 73 85

Road TypeResidential (190) 71.8 5 3 66 78U.S./Illinois Highways(40)

76.3 2 3 70 82

Interstate Highways (28) 87.3 4 3 81 93

Time of DayMorning Rush Hours (55) 75.7 5 4 68 84Noon Rush Hours (45) 74.8 5 3 69 81Evening Rush Hours (23) 80.9 7 5 71 91Non-Rush Hours (135) 76.2 2 1 74 78

Day of WeekWeekends (115) 79.1 4 3 73 85Weekdays (143) 73.5 5 3 68 79

Safety Belt Usage Rates in Illinois, July 2003

PUBLIC OPINION SURVEY OF “CLICK IT

OR TICKET” CAMPAIGN

Mehdi NassirpourIllinois Department of Transportation

SURVEY RESEARCH CENTER

• In the Spring of 2003, the Illinois Department of Transportation contracted with the Survey Research Office at the University of Illinois at Springfield to conduct two telephone surveys. Two of the surveys were conducted before and after a major seat belt initiative (that involved both media and enforcement-related activities that occurred during a time period surrounding Memorial Day Weekend, 2003. Interviewing for the “pre-test” (before) survey was conducted from April 25 to May 4, 2003, and interviewing for the “post-test” (after) survey was conducted from June 3 to 18, 2003.


OBJECTIVES

• To collect statewide data and information on public awareness and acceptance of the safety belt program during the “Click it or Ticket” program in Illinois at five points (before, during and after the campaign).

• To describe motorists’ opinions on safety belt use and related issues, controlling for demographics during the baseline and follow-up data collection periods.

• To identify key factors to assist the Division of Traffic Safety to measure effectiveness of the overall “Click it or Ticket” program and services in Illinois using the pre and post approach.


CLICK IT OR TICKET CAMPAIGN

• “Click It or Ticket” is a high visibility, massive enforcement effort designed to detect violators of Illinois traffic laws with special emphasis on occupant protection. An intense public information and education campaign was run concurrently with the enforcement blitz to inform the motoring public of the benefits of seat belt use and of issuing tickets for seat belt violations. The goal of the “Click It or Ticket” campaign is to save lives and reduce injuries resulting from motor vehicle crashes by increasing the safety belt usage rate in Illinois by at least 3-5

percentage points.


MEDIA/ENFORCEMENT

Several media events were held throughout the state on April 28, 2003 announcing the enforcement efforts planned for May 15 - June 1, 2003.

A total of 421 local agencies as well as Illinois State Police and Secretary of State Police agencies participated in the enforcement campaign and issued 46,434 citations. Over 26,085 man-hours were conducted a variety of enforcement efforts.


STRATIFIED RANDOM SAMPLE

Chicago Metro Downstate

City of Chicago

Chicago Area

Suburbs

North and

Central Illinois

Southern Illinois

Total Sample


SAMPLING METHODOLOGY

• Random samples of telephone numbers were purchased for each of the four stratification areas (City of Chicago, Chicago Suburban Counties, North/Central Illinois, and Southern Illinois). Each telephone number in the samples was called a maximum of six times, at differing times of the week and day. Within households, interviewers asked for the youngest licensed driver 75 percent of the time, because earlier experience showed that we under-represent younger drivers. In the other 25 percent of the time, interviewers asked for a licensed driver who was male/female (varying at random) and who had the next birthday. Replacements were accepted if that designated household member was not available.


DISTRIBUTION OF THE SAMPLES (Pre=560 and Post=623)

Pre-Test Post-Test

Chicago 113 160

ChicagoSuburbanCounty

228 177

DownstateCounties

219 286

North/CentralIllinois

137 166

SouthernIllinois

83 120


SAMPLING ERROR

• The sampling error for the statewide results is under +/- 5 percent (at the 95th confidence level) for both the pre-test survey (+/- 4.2%) and the post-test survey (+/- 4.0%). The error for subgroups is, of course, larger.


SAMPLE WEIGHTS

• The statewide results for both surveys have been weighted to arrive at a proper distribution by region and by gender.


The largest difference appears to be that for employment status, where the post-test survey has fewer full-time employees (37%, compared to 45% in the pre-test). The post-test respondents are also somewhat younger and have somewhat more minority respondents. The post-test sample also has somewhat more households with more than one person of driving age in the household.

DEMOGRAPHIC CHARACTERISTICS OF PRE AND POST SAMPLES


When Driving, how often do you wear your seat belt (composite of shoulder and lap)?

81.6%

9.5%4.6% 1.8% 2.5%

82.4%

9.1%4.7% 3.0% 0.8%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

All of the Time Most of theTime

Some of theTime

Rarely Never

Pre-Mobilization Post Mobilization

Data Source: Telephone Survey

N=528 for Pre Mobilization

N=576 for Post Mobilization

When was the last time you did not wear your seat belt when driving?

11.5% 11.4%5.9%

3.5%

59.7%

8.0%5.8% 4.7%

8.2%11.7%

8.4%

61.1%

0%

10%

20%

30%

40%

50%

60%

70%

Within the lastday

Within the pastweek

Within the pastmonth

Within the pastyear

More than ayear ago

(always wear)

DK/NA



In the past thirty days, has your use of seat belts when driving increased, decreased, or stayed the same?

5.4%0.5% 0.6%

10.4%

0.3%

88.8%

0.6%

93.5%

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

Increased Decreased Stayed the same DK/NA (Includes "newdriver")



According to Illinois state law, can police stop a vehicle if they observe a seat belt violation or do they have to observe

some other offense in order to stop the vehicle?

69.1%

19.6%

11.3%

70.9%

20.6%

8.4%

0%

10%

20%

30%

40%

50%

60%

70%

80%

Can stop just for seatbelt violation

Must see other offensefirst

DK/NA

Mobilization

Pe

rce

nt



In the past thirty days, have you seen or heard anything about the police setting up roadside safety checks where

they stop to check drivers and vehicles?

20.0%

77.4%

2.6%

44.7%

53.2%

2.1%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

Yes, have seen/heard No, have not DK/NA



In the past thirty days, have you seen or heard of any special effort by police to ticket drivers in your community for seat

belt violations?

12.2%4.3%

48.9% 48.1%

2.9%

83.5%

0%

10%

20%30%

40%

50%

60%70%

80%

90%

Yes, have seen orheard

No, have not DK/NA

Mobilization

Pe

rce

nt



Figure 21: In the past thirty days, have you seen or heard any messages that encourages people to wear their seat belts?

63.2%

35.7%

1.0%

82.2%

17.0%

0.8%0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

Yes, have seen/heard ofsuch

No, have not DK/NA



Do You Recall Hearing, or Seeing, the Following Slogans in the PastThirty Days?

Pre-Mobilization Post Mobilization Percent Change

Police in Illinois Arrest Drunk Drivers. 29.3% 23.6% -5.7%

Wanna drink and drive? Police in Illinois will show you thebars.

29.3% 48.0% 18.7%

Smart motorists always respect trucks. 10.9% 11.3% 0.4%

Help promote wreck-less driving. Don't hang out in the NoZone.

9.6% 9.0% -0.6%

Friends don't let friends drive drunk NA 89.0% NA

Stupid Driving Tricks /or/ Don't Pull a Stupid Driving Trick 5.0% 6.7% 1.7%

Click It or Ticket 66.7% 84.6% 17.9%

Buckle Up America 47.6% 53.2% 5.6%

Drive Hammered…Get Nailed! NA 30.1% NA

Children in Back 21.8% 23.6% 1.8%Cell phones save lives. Pull over and report a drunk driver. 39.2% 46.0% 6.8%

Drive smart, drive sober. 64.9% 66.9% 2.0%

Operation A-B-C 6.6% 5.1% -1.5%

You Drink, You Drive, You Lose NA 55.2% NA

Drink and Drive? Police in Illinois have your number. NA 22.3% NA


SAFETY BELT SURVEY DESIGN IN ILLINOIS Mehdi Nassirpour, Ph.D. Illinois Department of Transportation.

Documents

county c slide

cook county locations

dupage county locations

safety belt locations

idot slide

lake county locations

peoria county locations

champaign county locations