DOES A TOWBAR INCREASE THE RISK OF

NECK INJURY IN REAR-END COLLISIONS?A STUDY BASED ON A MERGER OF DANISH

ADMINISTRATIVE REGISTRIES

A N N E V I N G A A R D O L E S E N

H A R RY L A H R M A N N

T H E T R A F F I C R E S E A R C H G R O U P AT A A L B O R G U N I V E R S I T Y

D E N M A R K

The tow-bar

• A common sight in Denmark especially in rural areas

• Prevalence of 60% in the Motor Vehicle Registry in the period 2003-12

• From seller’s car database on the internet 37% in all of Denmark (2017)

• A count in typical parking lot gave a prevalence of 48% with 90% towbars

actually on the car (2017)

• Prevalence has dropped recently because of an increase in small cars (as

cars number two and else if mounting is not possible)

The study aim

• To investigate the association between the risk of neck injury in occupants

of the struck car in a rear-end collision

The registry merger

• The National Registry of Motor Vehicles: registration plates, towbar status

• Police reports: personal identification numbers, registration plates, accident

type

• The Central Person Registry: personal identification numbers, vital status,

nationality

• The National Hospital Discharge Registry: personal identification numbers,

diagnoses

• Socio-economic databases at Statistics Denmark: personal identification

numbers, income, highest attained education, socioeconomic status

• All information available at date of accident

• The personal identifier and the registration plate are the central merger

variables

The modern collision zone curls in rear-end collisions

• In order to absorb the force and protect occupants in the cabin

• Mounting of a stiff tow-bar frame is highly likely to inflate the curling

mechanism

• May increase the risk of neck injury in the struck car with a towbar

• Several ways to mount the tow-bar

• And now possible to demount when not using it

• All of which may ”mix” the impacts of a towbar registered

in a general Motor Vehicle Registry

Strong experimental evidence:

Krafft et al. (2000). AAP 32: 187-95.

• Two Volvo 240s one with and one without a towbar

• Car acceleration higher in towbar equipped car with peak at 9.6g

• Car acceleration in car without towbar had peak at 8.0g

• Dummy in the car with towbar experienced 33% higher peak acceleration in

lower neck, 8.9g

• Compared with 6.7g for dummy in the car without towbar

• Increased risk of 22% for long-term consequences of neck injury in struck

cars with a towbar (p<0.001)

• One other study found no effect Linder et al.??

• Few other sources cite Krafft et al.(2000) ??

Negative finding: no effect of tow-bar on neck injury

• Could be explained by measurement error on the information on tow-bar in

the Motor Vehicle Registry: imprecise exposure erases the effect if present

Counts, person-years, and neck injury rates. Public registry data from Denmark, 2003–2012

N N P-years IR Variable

Variable Persons Events

in

1000s

per

1000 95% CI significance

All 9370 1519 7.810 194.5 185.0 204.5

Tow-bar on struck car

Yes 5019 827 4.168 198.4 185.3 212.4 p < 0.0001

No 3244 575 2.652 216.8 199.8 235.3

Unknown 1112 117 0.990 118.2 98.6 141.6

Unadjusted

IRR=0.92; p=0.11

1-year prevalence

of 16%

Estimated hazard ratios in multivariate Cox proportional hazard regression.

Hazard ratios can be interpreted as incidence rate ratios. Public registry data from Denmark, 2003–

2012

Multivariate analysis

Hazard Robust Variable

Variable ratio SE 95% CI P-value significance

Tow-bar on struck car

Yes 0.915 0.0523 0.818 1.024 0.122 0.111

No 1.000

Unknown 0.820 0.0892 0.662 1.015 0.068

Adjusted for: gender, age, seat, income, education, accident

type, calendar year, weight of car, first registration year

Sex

Male 1.000 p < 0.0001

Female 1.538 0.0852 1.380 1.714 0.000

Age in years

0–17 0.936 0.2096 0.604 1.452 0.769 p < 0.0001

18–29 1.022 0.0766 0.882 1.183 0.775

30–39 1.000

40–49 0.904 0.0665 0.783 1.045 0.172

50–59 0.688 0.0613 0.577 0.819 0.000

60–69 0.546 0.0677 0.428 0.696 0.000

70+ 0.285 0.0659 0.181 0.448 0.000

Females have increased risk

Older persons tend to underreport?

Seat in struck car

Driver 1.000 p < 0.0001

Passenger 1.524 0.1302 1.289 1.801 0.000

Household income

<1st quintile 0.827 0.0740 0.694 0.986 0.034 p < 0.0001

1st–2nd quintiles 0.990 0.0782 0.848 1.155 0.895

2nd–3rd quintiles 1.000

3rd–4th quintiles 0.974 0.0785 0.832 1.141 0.746

>4th quintile 0.843 0.0774 0.705 1.010 0.063

Unknown 0.076 0.0363 0.030 0.194 0.000

Passenger seat associated with increased risk of neck injury

Poor and wealthy have reduced risk

Highest level of education

Primary education 1.019 0.1418 0.776 1.339 0.891 0.497

Upper secondary education 0.992 0.1563 0.728 1.350 0.957

Vocational Education and Training (VET) 0.948 0.1279 0.728 1.235 0.694

Qualifying educational programs 2.703 1.9514 0.657 11.127 0.168

Short-cycle higher education 1.000

Vocational bachelors’ programs 1.096 0.1569 0.828 1.451 0.522

Bachelors’ programs 0.929 0.2552 0.543 1.592 0.790

Masters’ programs 0.866 0.1586 0.605 1.240 0.431

PhD programs 0.592 0.4267 0.144 2.430 0.467

Unknown 0.812 0.1716 0.536 1.228 0.323

Educational effect explained by income level etc.

Accident type

Hit directly from the back when driving

straight ahead 1.000 0.069

Hit from the back when turning right 0.730 0.1087 0.545 0.977 0.035

Hit from the back when turning left 0.943 0.0658 0.822 1.081 0.397

Calendar year

2003–2004 1.000 0.481

2005–2006 0.988 0.0770 0.848 1.151 0.880

2007–2008 0.899 0.0728 0.767 1.054 0.188

2009–2010 0.979 0.0835 0.828 1.157 0.806

2011–2012 0.882 0.0817 0.735 1.058 0.175

Reduced risk when turning right

No significant effect of calendar year

Weight of struck car in kilograms

500–1000 0.996 0.0681 0.871 1.139 0.950 0.985

1000–1500 1.000

1500–2000 1.019 0.1100 0.825 1.259 0.861

2000+ 0.853 0.2513 0.479 1.519 0.589

Unknown 1.029 0.6233 0.314 3.373 0.962

First registration year of struck car

1966–1989 0.946 0.1076 0.757 1.182 0.623 0.495

1990–1994 1.035 0.0940 0.867 1.237 0.701

1995–1999 0.962 0.0739 0.828 1.119 0.618

2000–2004 1.000

2005–2009 0.839 0.0784 0.699 1.008 0.060

2010–2015 0.827 0.1608 0.565 1.211 0.328

Unknown 1.079 0.6575 0.327 3.562 0.900

No effect of weight and first registration year of struck car

Conclusion

• 16% of occupants of struck cars were diagnosed with neck injury by doctors

in hospitals/emergency rooms after rear-end collisions

• No effect of tow bar on the struck car in large study of 9370 occupants

• Measurement error might induce result

• Count of cars in typical parking lot demonstrated that 90% of tow-bars were

actually on the car (and not taken off)

• More crash test needed in order to corroborate registry finding

• Could the tow-bar be associated with neglectible risk of neck injury in

modern cars

• Law of physics does somehow rule

What we learnt from this merge

• Remarkable possibilities for registry merge in Denmark and Scandinavia –

which is of course well-known

• Merge by registration plate in police reports and Motor Vehicle Registry

possible

• Merge by personal identifier in police reports and National Discharge

Registry (hospital registry) possible

• Denmark is only a novice regarding registry studies

Strengths and limitations of registry studies

• Complete populationbased studies with many cases (likely a high power)

• Facilitate assessment of various hypotheses such as mix between different

personal injuries and diagnoses and hospital courses

• But -

• Measurement error such as on information of tow-bar status in the Motor

Vehicle Registry

• Lack of information on confounders such as (precise) personal

characteristics, attitudes and psychological profiles

• Missing values in some variables ”unknowns”

Experiences with registry data in Scandinavia?

• Which kinds of merges are possible in the different countries?

• Experiences with different sources such as police reports with registry data?

• Experiences with various sorts of sources of error?

• How to overcome the limitations: sensitivity analyses and multiple

imputation or ?

• Other issues?