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

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Presentation on theme: "SAFETY BELT SURVEY DESIGN IN ILLINOIS Mehdi Nassirpour, Ph.D. Illinois Department of Transportation."— Presentation transcript:

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

2 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

3 Census Tracts Road Type HVR LVRHVRLVRHVRLVR City of Chicago 77 Communities Cook C No Chicago 30 Townships Other Counties 101 Counties UrbanRural Road Type SAMPLE DESIGN SUMMARY IN ILLINOIS Stage 1 Stage 2 Stage 3

4 ALLOCATION OF SAMPLE SITES Total Sample (258 Sites 12 Counties (166 Sites) Cook County (92 Sites) Cook County No Chicago (41 Sites) City of Chicago (51 Sites)

5 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

6 SAFETY BELT LOCATIONS IN IL MCHENRY COUNTY LOCATIONS (6) LAKE COUNTY LOCATIONS (31) KANE COUNTY LOCATIONS (14) WILL COUNTY LOCATIONS (18) CHICAGO LOCATIONS (49) COOK COUNTY LOCATIONS (40) DUPAGE COUNTY LOCATIONS (42) CHAMPAIGN COUNTY LOCATIONS (9) ROCK ISLAND CO. LOCATIONS (7) PEORIA COUNTY LOCATIONS (9) MONTGOMERGY COUNTY LOCATIONS (2) ST. CLAIR COUNTY LOCATIONS (14) Division of Traffic Safety at IDOT

7 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 Division of Traffic Safety at IDOT

8 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. Division of Traffic Safety at IDOT

9 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. Division of Traffic Safety at IDOT

10 Weighting Procedures W ctr = 1/P ctr P ctr = P c. P t/c. P r/ct cidentifies a sample of county tidentifies a sample of tract ridentifies a sample of road site P c probability of selecting county c P t/c probability of selecting tract t within selected county c P r/ct probability of selecting road r, conditional to the tract and county Division of Traffic Safety at IDOT

11 FORMULA FOR ESTIMATING SAFETY BELT USAGE RATE R = Safety belt usage rate W ctr = weight associated with road r in tract t in county c; X ctr = number of safety-belted front seat occupants of passenger cars and pick-up trucks observed in county c, tract t, and road site r; and Y ctr = number of front seat occupants observed in county c, tract t, and road site r. Division of Traffic Safety at IDOT

12 FORMULA FOR ESTIMATING SAFETY BELT USAGE RATE. F ctr = adjustment for lanes not observed in road site r, tract t in county c; L ctr = total number of lanes in road site r, tract t in county c; and O ctr = total number of observed lanes in road site r within tract t in county c

13 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. Division of Traffic Safety at IDOT

14 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. Division of Traffic Safety at IDOT

15 VARIANCE ESTIMATION FORMULA. (Y ijkl, X ijkl, Y ijk2, X ijk2,...., Y ijkp, X ijkp ) 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

16 Safety Belt Usage Rates in Illinois, July 2003

17 PUBLIC OPINION SURVEY OF CLICK IT OR TICKET CAMPAIGN Mehdi Nassirpour Illinois Department of Transportation

18 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. Division of Traffic Safety at IDOT

19 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. Division of Traffic Safety at IDOT

20 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. Division of Traffic Safety at IDOT

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22 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. Division of Traffic Safety at IDOT

23 STRATIFIED RANDOM SAMPLE Chicago Metro Downstate City of Chicago Chicago Area Suburbs North and Central Illinois Southern Illinois Total Sample Division of Traffic Safety at IDOT

24 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. Division of Traffic Safety at IDOT

25 DISTRIBUTION OF THE SAMPLES (Pre=560 and Post=623) Division of Traffic Safety at IDOT

26 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. Division of Traffic Safety at IDOT

27 SAMPLE WEIGHTS The statewide results for both surveys have been weighted to arrive at a proper distribution by region and by gender. Division of Traffic Safety at IDOT

28 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 Division of Traffic Safety at IDOT

29 Data Source: Telephone Survey N=528 for Pre Mobilization N=576 for Post Mobilization

30 Data Source: Telephone Survey

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