STATEWIDE OBSERVATIONAL SURVEY OF SAFElY BELT AND MOTORCYCLE HELMET USE IN TEXAS Prepared By KN. Womack VJ. Pezoldt

Transcription

1 STATEWIDE OBSERVATIONAL SURVEY OF SAFElY BELT AND MOTORCYCLE HELMET USE IN TEXAS Prepared By KN. Womack VJ. Pezoldt Prepared For Traffic Operations Division Texas Department of Transportation Jeanne Swanson, Project Manager In Cooperation With National Highway Traffic Safety Administration U.S. Department of Transportation THE TEXAS A&M UNIVERSITY SYSTEM COLLEGE STATION, TEXAS

2 STATEWIDE OBSERVATIONAL SURVEY OF SAFE1Y BELT AND MOTORCYCLE HELMET USE IN TEXAS BACKGROUND As of December 1, 1985, Texas has had a mandatory use law (MUL) that requires drivers and front seat passengers to use safety belts, with drivers held responsible for passengers under 15 years of age. The MUL applies to occupants of passenger cars and pick-up trucks of less than 3/4 tons. It exempts persons for medical reasons (with a written physician's statement) and postal employees in box-to-box mail delivery. The fine for noncompliance is $25 - $50. The Texas MUL allows for primary enforcement. Additionally, since September 1, 1989, Texas has required all motorcycle drivers and passengers (including moped and scooter riders) to wear helmets. The mandatory motorcycle helmet use law of 1989 is a reinstatement of a 1968 law that applied to all motorcyclists. The 1968 mandatory use law had been amended in 1977 to apply only to those under age : The Texas Transportation Institute conducted a probability-based survey to estimate safety belt and motorcycle helmet use in Texas. The purpose of the survey was to determine eligibility for participation in the grant program in Section 153 of Title 23, United States Code. This Section allowed the Secretary of Transportation to award grants to states that have safety belt and motorcycle helmet use laws during the first year of the program, and to award grants during the second the third years of the program to states that achieved specified rates of compliance. The National Highway Traffic Safety Administration (NHTSA) issued guidelines and provided funding for the observational survey described herein. The following is a description of the sample design, survey procedures, and results of the Texas survey conducted from May 15 through June 15,

3 THE SAMPLE DESIGN NHTSA guidelines required that at least 85% of a state's population must be eligible for inclusion in the sample. All 254 Texas counties were listed in descending order of population (based on 1990 census data). The percentage of the State population in each county and the cumulative percentage of the State population were calculated. The 60 largest counties account for 85.08% of the State population (Table 1). These 60 counties comprise the eligible geographic locations for the survey and define the population of Texas for purposes of the sampling plan. Sample size determination A previous Texas statewide seat belt use survey conducted by the Washington Consulting Group (1991) employed a probability-based sample. The data from that survey were used to determine the sample size for the present survey plan. Pertinent information from the prior survey is provided in Table 2. Also shown in Table 2 are the calculations used to determine the proposed sample size of 248 observation sites. An observation site was defined as a controlled intersection from which observations of belt and helmet use were made. Allocation and Selection of Observation Sites 1. Allocation and Selection of Counties The primary sampling units were counties. Based on the guidance provided by NHTSA, 22 counties were selected. Selection of counties that were included in the sample employed a two stage procedure. First, to decrease the probability that a few very large counties would be selected to the exclusion of the many smaller population counties, the six largest counties in Texas were selected with certainty for inclusion in the sample. These counties account for about 57% of the state population (as defined by the 60 eligible counties described above). Second, in order to assure wide geographic coverage, the selection of the remaining 16 counties was stratified by region within the state. This stratification is based on ten Uniform State Services Regions (USRs) in Texas. The number 2

4 Table 1. Population of 60 Largest Texas Cculties 1990 Texas Population = Largest Counties Population = (85.08X of total) Proportion CLJTUlative Rank PoJ2!:!lation of State Proportion 1 Harris County Dallas County Bexar County Tarrant County El Paso County Travis County Hidalgo County Nueces County Denton County Collin County Cameron County Jefferson County Fort Bend County Lubbock County Galveston County Brazoria County Bell County Mclennan County Montgomery County Smith County Williamson County Webb County Wichita County Brazos County Taylor County Ector County Midland County Gregg County Tom Green County Potter County Johnson County Grayson County Randall County Ellis County so Bowie County Orange County Victoria County Angelina County Hays County Guadalupe County Parker County Hunt County Coryell County San Patricio County Henderson County Harrison County Nacogdoches County Liberty County Kaufman County Comal County Walker County Anderson County Lamar County Rusk County Hardin County Cherokee County Starr County Wharton County Navarro County :... :

5 WCG Seat Belt Survey Table 2. Determination of Sample Size No of Sites: No of Obsetvations: Mean Seat belt use per site: Standard Deviation: Standard error of mean: %Precision [(std errmean/mean) X 100]: , % 20.04% 1.08% 1.71% Sample size (number of observation sites) for TTl statewide survey where: = sample size = variance (est used, s from WCG) = relative degree of precision required, ± 2.5% = 1.96.os n = /(2.5/1.96) n = (24 7) Number of observation sites for TTl statewide survey = 248 of counties allocated to each USR is proportional to the USR's proportion of the State population (less the population of the counties selected with certainty). The 16 counties were selected with probabilities equal to each county's proportion of the population within its region. Table 3 lists the 22 counties selected for inclusion in the sample. Figure 1 indicates the geographic distribution of the 22 sample counties. 4

6 Table Counties Selected for Sample No. of No. of Co. No. of Tracts intersections ~ USR Name Tracts Selected selected Pooulation Harris ,818, Dallas ,852,810 Selected 29 8 Bexar ,185,394 { with Tarrant ,170,103 Certainty El Paso , Travis , Lubbock ,636 ljl II Wichita , Collin , Ellis , Kaufman , Cherokee ,049 Selected with Harrison ,483 Probability Jefferson ,397 { Proportional to Fort Bend ,421 USR Population Galveston , Bell , Brazos , Cameron , Hidalgo , Webb , Tom Green , ,910,021

7 Figure l Geographic Distribution of 22 Sample Counties 6

8 2. Allocation and Selection of Census Tracts One hundred twenty-four (124) census tracts were allocated among the 22 selected counties. In order to assure that within county variance could be determined, two tracts were allocated to each county. The remaining 80 tracts [124- (2x22)] were allocated to the sample counties proportionally, based on the total number of census tracts in the sample counties. The probability of selecting a given tract in each county was proportional to the square root of the tract's population. The number of tracts selected from each county is indicated in Table Allocation and Selection of Intersections for Observations Intersections that represented all primary and collector roads within each census tract were identified. A simple random selection of two intersections was made from each tract, resulting in a total of 248 intersections, as indicated in Table 3. The southwest comer of each tract was chosen (at random from the four cardinal directions) to serve as the starting point for selecting the intersections for inclusion in the survey. Moving in a clockwise direction, the third and sixth intersections were selected. The probability of selecting a given intersection within a census tract was equal to the reciprocal of the number of intersections in the tract (1/ number of intersections). Alternate intersections were selected using the same method to provide backup sites for those instances in which the primary site was inappropriate for data collection (see Data Collection Procedures below). 4. Allocation of Observation Days of Week and Times of Day Observations were conducted for one hour at each selected site. Twelve observation periods were employed, with start times at five-minute increments from 7:00am through 6:00 pm. All days of the week, Monday - Sunday were eligible. Ideally, an observation period start time and day of week would be randomly selected from all the eligible periods and assigned to each site. Logistic consideration made such random assignment problematic. Therefore, for census tracts within counties, sample times were clustered in three hour blocks and sample days were clustered in two day blocks. Statistical notation of the probabilities associated with selection of observation sites, calculation of belt use estimates, and variance estimates are provided in Attachment A 7

9 Data Collection Protocol Each observer was instructed to follow the protocol described in the following sections. Observers were Texas A&M University students who were trained and monitored by the TTl study staff. A sample data collection form is provided as Appendix B. Schedule Observers will follow a schedule that indicates the day, start time, intersection name, site number, and direction of travel for observations. Substitutions of days and start times are not acceptable because sites have been selected and assigned to days of the week and times of the day by probability methods. H an assigned intersection is not acceptable for observation, the surveyor should relocate to Alternate Site A, or to Alternate Site B if necessary. Unacceptable intersections are: 1) under construction to the extent that traffic is diverted, 2) uncontrolled intersections; i.e., no traffic signal, stop sign, or yield sign, 3) intersections with no place to stand where traffic can be observed, or 4) intersections where personal safety is at risk. Examples of intersections where personal safety is at risk might be a gang corner, a drug dealers' corner, or an intersection occupied by a prostitute. Intersections that would not be considered a threat to personal safety include those that are in a "bad" part of town, those that are isolated, or those where "undesirable" people are within view. Selections of Alternate Sites should be documented on the observation report form in the Comments section. Freeway site observations should be made at the first intersection after the exit ramp, unless otherwise noted. 8

10 It is important to arrive at each site at least 10 minutes early to a) determine that the intersection is acceptable, and b) to determine the ratio of vehicles to be observed (see Section III). If, for any reason, you are unable to comply with the schedule, you should notify the project director at Til ( ). Vehicles To Be Observed Eligible vehicles include passenger cars and pickup trucks. Exclude buses, large trucks (one ton and larger), recreational vehicles and postal delivery vehicles. Observation Procedure The following procedures will be used in conducting observations of safety belt use. The first counter button on the left should be used for drivers and front seat passengers sitting by the window who are wearing shoulder harnesses. The second counter button from the left should be used for drivers and front seat passengers sitting by the window who are not wearing shoulder harnesses. The third counter button from the left should be used for pickup truck drivers and passengers sitting by the window who are wearing shoulder harnesses. The fourth counter button from the left should be used for pickup truck drivers and passengers sitting by the window who are not wearing shoulder harnesses. Pickup trucks are defined as vehicles with a cab and a bed. One ton and larger trucks are not eligible for the survey. Do not count any trucks with dual wheels or a dual cab. Only shoulder harnesses, properly used, should be recorded. If you notice a lap belt in use without a shoulder harness, it should not be recorded as using a safety belt. 9

11 Determine how many vehicles in every lane in the assigned direction it is possible for you to observe. In most situations it should be possible to observe every vehicle. However, if traffic is moving too fast to observe every vehicle, you should observe a ratio, i.e., one of two, one of three, etc. This pattern must be followed for the entire observation period and recorded in the vehicle ratio blank of the Observation Summary. H you are unable to determine shoulder harness use for a driver or passenger, a tick mark should be made in the right hand margin of your data collection form. Upon completion of the site, you must collect data for an additional number of vehicles equal to the number of "unsures" observed during the observation period. Children riding in car seats in the front seat position by the door should be counted as wearing shoulder harnesses. Motorcyclists with and without helmets should be recorded during the observation using tick marks in the space provided on the observation form. A tick mark should be used for each rider (driver and passenger). Bicyclists with and without helmets should be recorded during the observation using tick marks in the space provided on the observation form. A tick mark should be used for each rider (driver and passenger). At the end of the observation period, obtain from the counters and record on the form the number of persons observed using shoulder belts and the number observed not using shoulder belts. H no eligible vehicles were observed during the observation time period, record zero for both belted and not belted lines. Observers were provided with schedules in advance that specified: 1) the location of the intersection to be observed; 2) the direction of traffic to be observed; and 3) the day of the week and hour to start their observation. SURVEY RESULTS As indicated in Table 4, 67,065 front seat occupants of passenger vehicles were observed during the statewide survey, of which 49,251 (69.1 percent) were restrained. Additionally, 277 of 285 motorcycle riders observed (97.0 percent) were wearing helmets. The estimated error and confidence intervals are shown in Table 4. 10

12 I Table 4. Statewide Results I Estimated mean safety belt use for Texas Standard error of the estimate 2.75 Lower 95% confidence limit Upper 95% confidence limit Front seat occupants restrained 49,251 Front seat occupants observed 67,065 Estimated Motorcycle Helmet Use Helmetted Riders 277 Observed Riders

13 APPENDIX A 1. Notation and Probabilities of Selection of Sampling Units 2. Estimates of Proportion of Belt Use 3. Standard Errors of the Estimates 12

14 1 Notation and Probabilities of Selection of Sampling Units NOTATION i = County index J = Census track index K = Site (census block) index S nnhols No. of Counties Certainty Group Ponulation Sample Remaining Counties Population Sample N n = N n No. census tracts in County i M.* I m.* I No. of Sites in Tract j, County i b,,.. = 2 B.. * IJ b,,..*= 2 Population for Site K, tract j. County i (May not be known) P.. ~ lj-. Population for tract j, County i pij B.. _!J.:... =L_ pijlc. K=l B; P*.. = L P.. lc. IJ' K=l " Population for County i M; m; Pi.. = _L_ pij j=l pi~ = _L_ P.. I}' j=l 13

15 Population for group of counties P... = N L_ P... i=l in region r N. =1:_P.. i=l 1 p... = population of region (r), not including the population of any cenainty counties in the region p = Total population for all remaining counties (60 minus cenainty) Prob. selecting County i rr; = P/./P... Pro b. selecting census tract j, given sa. in county i L_~. 1 v ]=... ij Mi L_{P; j=l 14

16 SAIVlPLING & ESTIMATION NOTATION i = County index J = Census track index K = Site (census block) index Svmhols No. of Counties Certainty Group Population Sample Remaining Counties Population Samnle N n=n n No. census tracts in County i M.* I m.* I No. of Sites in Tract j, County i b.,..*= 2 Population for Site K, tract j, County i (May not be known) Population for tract j, County i pij B..!L =L_ P.'l IJ K"'l P*.. I)" = B: _L_ K=l pijk. Population for County i pi Mi = L_ pij j=l pi~ = mi _L_ j=l pij 15

17 II STEP 1 ESTIMATE PROPORTION FOR TRACT J. COUNTY I Certaintv Group 2 = L_ P..,_ Y..,_ K= 1 '1"' '1"' 2 E P,IJ -.c K=1 2 L_ k= 1 tiik Y;;t 2 E -1-tijk unknown Counties within Region (r) 1\ r; 2 L_ P-~t K=l = 2 L_ pj;j: K=1 r. k I) IJ 2 16

18 STEP 2 ESTTI\fATE PROPORTION FOR COUNTY I Cert.aintv Group = 1 ml A -P.. - ~ v Y.. L...J I} m p j"=l rr,,; j i ' Counties within region (r) j=1 mi pi STEP 3 ESTTI\fA TION OF PROPORTION FOR GROUP Cert.aintv Group Counties within region (r) A 1 = - y P... N N mi A I)" Y.. = - L_ L_ I) 1,.. Y. m; P... 1 j=1 II.. IJ - - P.. - L_ P. ~ 1 ' P... n i=l rr; = 1 n p. n L_ 1 i=1 m; I( m; p.. ~ I]" rj; j=1 II;; L_ 1\ y = 1 pr:gions I: 1\ P... - y r 10 - L_ = w~ r=l r Yr where w, P.~. = p 17

19 111 STEP 4 ESTIMATE FOR ENTIRE STATE(= 60 COUNIIES> Certainty Counties: = (.J:... ) 2 t [ " 1 - { L ( p ij yij ) 2-Mi (pi. ~) } ] P... i l Mi (Mi 1) Ilij 1\ s. e. ( Y) =/ v ( f) Counties within re!!ions (r) v (Y') = L (P:f (-1 I 1 L (pi_ y - p y-l regions p ) p ) n (n' -1) i=1 rr; ) n n p )l( )2 [L rp 1 r r ( = P : ;:_ n (~' -1) i=1 l ~ ) ' 1 -n (P... Y")2_\ State ( = "35% state") 10 s.e. (Y) = w 2 v(y) = -Et (1-W) v, (Y"), Where: W---- P... P... +p 18

20 APPENDIX B Data Recording Form 19

21 Texas Safety Belt Observation Study.erver: I= Tim-e: IATION SITE DESCRIPTION! Number: Type of Control: lnty: Number of Lanes: secting Street Names: Posted Speed: Weather Conditions: llments: Motorcycle Helmet Yes No SUMMARY: TOTAL BELTED (CARS): TOTAL NOT BELTED (CARS): TOTAL BELTED (PICK_UPS): TOTAL NOT BELTED (PICKUPS): VEHICLE OBS. RATIO: TOTAL e of Control lour-way Stop Sign wo-way Stop Sign ignal =Yield Sign IYCLE HELMETS Posted Speed 1 =Up to and including 30 MPH 2=31 to 50 MPH 3=Faster than 50 MPH Weather Condition 1 =Clear 2=Rain 3=Fog I NO: II 20