Does a general impairment test exist, that specifically would be useful for the context of driving? We know that  alcohol negatively influences a driver's ability to drive. Governments typically combat this by making drunk driving illegal, and use the concentration of alcohol in breath or blood as the definitive measure as to whether a crime has been committed or not. 

In this sense - alcohol in the blood/breath is merely a proxy for impairment, but one that is quite reliable - when one is drunk they're going to show alcohol in the blood stream, this is able to be accurately measured.  

There are other ways a driver can be impaired as well - quite commonly through sleepiness/fatigue, but also other drugs, or distraction (driving while texting), or emotional distress. 

The prospect of 'drugged driving' comes up quite commonly in discussions about legalising cannabis. Where there is a reliable test of alcohol impairment - it's not quite the same for cannabis and other drugs - the problem being that saliva tests can show the presence of the drug long after the person has sobered up. A blood test might be viable - but that's perhaps not practical, in terms of getting a cop to administer to a driver. 

With fatigue, it's even more problematic - there's no drug to detect at all. 

In light of this - has any general impairment test - that perhaps tests reaction times or some other metric, that can be administered at a roadside, been developed?
Based in Madison, Wisconsin, CTC &amp; Associates provides technical communications services for the transportation sector, and together with the Minnesota Department of Transportation (MnDOT), they put together a report on how to monitor driver fatigue:


  to serve as a synthesis of pertinent completed research to be used for further study and evaluation by MnDOT. This [Transport Research Synthesis (TRS)] does not represent the conclusions of either CTC &amp; Associates or MnDOT. (MnDOT, 2015)


The following (apart from ESS - see below) are some of the studies presented in the MnDOT report.

In-vehicle Technologies

Balkin, et al. (2011) talks about technologies which can detect or predict driver fatigue.


  Technologies that objectively detect or predict operator fatigue may be used to effectively complement or even supplant organizational or regulatory approaches. Over the past decade and a half, there have been considerable advances in relevant technologies, including onboard devices that monitor drivers’ state or level of performance as well as devices that predict fatigue in advance of a work cycle or trip.


Barr, Popkin and Howarth (2009) also reviews and discusses unobtrusive, in-vehicle, real-time drowsy driver detection and fatigue monitoring/alerting systems.


  The study focuses on recent developments in two areas:
  
  
  Mathematical models that predict operator alertness and performance at different times. Factors
  involved in the assessment include the amount of sleep obtained or missed, circadian factors (the 24-
  hour cycle of physical, mental and behavioral changes) and workload.
  Vehicle-based monitoring technologies that assess behavioral characteristics of the driver such as eye
  gaze, eye closure, head position and movement, and heart rate.
  


Roadside and other Out-of-vehicle technologies

Fitness for Duty

There is the Karolinska Sleepiness Scale [KSS] (Åkerstedt &amp; Gillberg, 1990; Kaida, et al., 2006) and there is the Epworth Sleepiness Scale [ESS] (Johns, 1991).  Both are self-report scales, frequently used in studies measuring subjective sleepiness, that measure the individual’s drowsiness.  The KSS is a 9-point verbally anchored scale going from ‘extremely alert’ to ‘extremely sleepy–fighting sleep’, whereas the ESS uses a 4-point scale from never doze to high chance of dozing.

The Epworth Sleepiness Scale (ESS) (named after The Epworth Sleep centre in Melbourne, Australia) is used by doctors here in the UK to point toward the possibility of sleep disorders such as Sleep Apnoea. You can see an example PDF of the questionnaire at the Warrington and Halton NHS Hospitals website. It is a self-evaluation questionnaire devised to determine general sleepiness observed by the individual based on usual chances of dozing off or falling asleep while engaged in eight different activities.

Not wishing to add any validity to the system, I thought I would mention that Optalert talks about a testing system they developed themselves and are promoting, called the Johns Drowsiness Scale (JDS).  It was developed by Optalert’s Founding Director, and developer of ESS, Dr Murray Johns, and comparing it to ESS, and Blood Alcohol Concentration Readings, they say that


  With 30 Per Cent Of Road Fatalities Attributed To Drowsy Driving, At Optalert We Believe The Johns Drowsiness Scale (JDS™) Will, In The Near Future, Become As Important As Blood Alcohol Concentration (BAC) Readings. (Source: JDS vs BAC - Optalert)


Whilst assessing the use of a simple balance task for assessing fitness for duty, Sargent et al. (2012) states:


  results indicate that postural balance may be a viable tool for assessing fatigue associated with time of day, but may not be useful for assessing fatigue associated with extended hours of wake.


Roadside Testing

A pilot study was conducted (Forsman, et al., 2014) and it states that the method put together for the pilot...


  further developed, may provide a drowsiness test for roadside surveillance.


Further Reading


  MnDOT. (2015) Monitoring Motor Vehicle Driver FatigueRetrievable form http://dotapp7.dot.state.mn.us/projectPages/pages/projectDetails.jsf?id=35917&amp;type=DOCUMENT


Plus Chapters 9 &amp; 10 of:


  National Academies of Sciences, Engineering and Medicine. (2016). Commercial Motor Vehicle Driver Fatigue, Long-Term Health, and Highway Safety. Washington (DC): National Academies PressDOI: 10.17226/21921 NBK: 384966 [Free PDF]


References

Åkerstedt, T., &amp; Gillberg, M. (1990). Subjective and objective sleepiness in the active individual. International Journal of Neuroscience, 52(1-2), 29-37.DOI: 10.3109/00207459008994241 PMID: 2265922

Balkin, T. J., Horrey, W. J., Graeber, R. C., Czeisler, C. A., &amp; Dinges, D. F. (2011). The challenges and opportunities of technological approaches to fatigue management. Accident Analysis &amp; Prevention, 43(2), 565-572.DOI: 10.1016/j.aap.2009.12.006

Barr, L., Popkin, S., &amp; Howarth, H. (2009). An evaluation of emerging driver fatigue detection measures and technologies (No. FMCSA-RRR-09-005). United States: Federal Motor Carrier Safety Administration.Retreivable at https://rosap.ntl.bts.gov/view/dot/34394

Johns, M. W. (1991). A New Method for Measuring Daytime Sleepiness: The Epworth Sleepiness Scale. Sleep, 14(6), 540-545.DOI: 10.1093/sleep/14.6.540

Forsman, P., Pyykkö, I., Toppila, E., &amp; Hæggström, E. (2014). Feasibility of force platform based roadside drowsiness screening–A pilot study. Accident Analysis &amp; Prevention, 62, 186-190. DOI: 10.1016/j.aap.2013.09.015

Kaida, K., Takahashi, M., Åkerstedt, T., Nakata, A., Otsuka, Y., Haratani, T., &amp; Fukasawa, K. (2006). Validation of the Karolinska sleepiness scale against performance and EEG variables. Clinical Neurophysiology, 117(7), 1574-1581.DOI: 10.1016/j.clinph.2006.03.011

MnDOT. (2015) Monitoring Motor Vehicle Driver FatigueRetrievable form http://dotapp7.dot.state.mn.us/projectPages/pages/projectDetails.jsf?id=35917&amp;type=DOCUMENTTRSs are also searchable at http://dotapp7.dot.state.mn.us/projectPages/pages/homepage.jsf

Sargent, C., Darwent, D., Ferguson, S. A., &amp; Roach, G. D. (2012). Can a simple balance task be used to assess fitness for duty?. Accident Analysis &amp; Prevention, 45, 74-79.DOI: 10.1016/j.aap.2011.09.030