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Adjusted Odds Ratios Between Drug Use and Crash Risk

The table below shows data from the US National Highway Traffic Safety Association’s latest study on the hazards of substance use and driving. The study found that

[d]rivers testing positive for THC were overrepresented in the crash-involved (case) population. However, when demographic factors (age and gender) and alcohol use were controlled, the study did not find an increase in population-based crash risk associated with THC use.

Further, the study found that

cannabis did not exacerbate the dangers of drinking and driving as had previously been believed, finding there was no difference in crash risk for marijuana (THC)-positive drivers who were also positive for alcohol than for marijuana (THC)-positive drivers with no alcohol, beyond the risk attributable to alcohol. Further analyses examined the potential interaction between drug use and breath alcohol concentration (BrAC). No statistically significant interaction effect on crash risk was found between any drug class or drug category and BrAC level.

Adjusted Odds Ratios Between Drug Use and Crash Risk

(Adjusted for Demographic Variables and Alcohol Use)

Drug of Interest Adjusted Odds Ratio 95% CI* P Value
THC (Marijuana) 1 0.83 – 1.22 0.98
Antidepressants 0.86 0.56 – 1.33 0.5
Narcotic Analgesics 1.17 0.87 – 1.56 0.3
Sedatives 1.19 0.86 – 1.64 0.29
Stimulants 0.92 0.70 – 1.19 0.51
Illegal Drugs 0.99 0.84 – 1.18 0.99
Legal Drugs 1.02 0.83 – 1.26 0.83
The risk of crash involvement for each category and class of drug is compared to the crash involvement rate for drug-negative drivers. An odds ratio of 1.00 means the crash involvement rate is the same.
*(CI = Confidence Interval)
Source: NHSTA (http://www.nhtsa.gov/staticfiles/nti/pdf/812117-Drug_and_Alcohol_Crash_Risk.pdf)

Although this table demonstrates that most drugs do not impact highway safety, readers and researchers should be aware that alcohol is not listed in these tables. Use of alcohol was found to increase the risk of accidents even when drivers blood alcohol content remained below the legal limit. Notice that cannabis has a risk of exactly 1, meaning cannabis users posed the same risk to traffic safety as drivers with no substances in their systems. Some substances appear to lower the risk of accident, which is not surprising in the case of stimulants but perhaps counter-intuitive with respect to illegal drugs as a class. Not surprisingly, narcotics and sedatives pose the greatest threat out of all the substances in the table.

Booze

Just for grins, let’s take a look at the data for alcohol.

BrAC Relative Risk
Unadjusted and Adjusted for Age and Gender

BrAC Unadjusted Risk Adjusted Risk (Age and Gender)
0 1 1
0.01 0.51 0.54
0.02 0.82 0.85
0.03 1.17 1.2
0.04 1.57 1.6
0.05 2.05 2.07
0.06 2.61 2.61
0.07 3.25 3.22
0.08 3.98 3.93
0.09 4.83 4.73
0.1 5.79 5.64
0.11 6.88 6.67
0.12 8.11 7.82
0.13 9.51 9.11
0.14 11.07 10.56
0.15 12.82 12.18
0.16 14.78 13.97
0.17 16.97 15.96
0.18 19.4 18.17
0.19 22.09 20.6
0.20+ 25.08 23.29
Note: (Relative to BrAC = .00)

Notice that at very low levels people overcompensate for their impairment and are actually more careful than sober drivers. However, by the time BrAC (Breath Alcohol Content) reaches .03% (or one drink for a man less than 140 pounds or one drink for a woman weighing less than 180 lbs) driving is impaired. After two drinks most people (i.e. everyone under 240 lbs) are at least twice as dangerous as a sober driver and may be up to five times as dangerous (again depending on gender and weight). Here is a chart mapping number of drinks to BrAC based on gender and body weight:

MaleFemale Approximate blood alcohol percentage (by vol.)[3]One drink has 0.5 US fl oz (15 ml) alcohol by volume
Drinks Body weight
40 kg 45 kg 55 kg 64 kg 73 kg 82 kg 91 kg 100 kg 109 kg
90 lb 100 lb 120 lb 140 lb 160 lb 180 lb 200 lb 220 lb 240 lb
1 –0.05 0.040.05 0.030.04 0.030.03 0.020.03 0.020.03 0.020.02 0.020.02 0.020.02
2 –0.10 0.080.09 0.060.08 0.050.07 0.050.06 0.040.05 0.040.05 0.030.04 0.030.04
3 –0.15 0.110.14 0.090.11 0.080.10 0.070.09 0.060.08 0.060.07 0.050.06 0.050.06
4 –0.20 0.150.18 0.120.15 0.110.13 0.090.11 0.080.10 0.080.09 0.070.08 0.060.08
5 –0.25 0.190.23 0.160.19 0.130.16 0.120.14 0.110.13 0.090.11 0.090.10 0.080.09
6 –0.30 0.230.27 0.190.23 0.160.19 0.140.17 0.130.15 0.110.14 0.100.12 0.090.11
7 –0.35 0.260.32 0.220.27 0.190.23 0.160.20 0.150.18 0.130.16 0.120.14 0.110.13
8 –0.40 0.300.36 0.250.30 0.210.26 0.190.23 0.170.20 0.150.18 0.140.17 0.130.15
9 –0.45 0.340.41 0.280.34 0.240.29 0.210.26 0.190.23 0.170.20 0.150.19 0.140.17
10 –0.51 0.380.45 0.310.38 0.270.32 0.230.28 0.210.25 0.190.23 0.170.21 0.160.19
Subtract approximately 0.01 every 40 minutes after drinking.   (Source: WikiPedia)
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