New research shows why “Don’t drive under 5” matters

By Dr Steven Trawley

More than 400,000 Australians manage their diabetes with insulin. This puts them at risk of low glucose levels (hypos). Guidelines for those who drive focus on avoiding hypos. Drivers with insulin-treated diabetes are advised to start their journey only when their glucose is above 5 mmol/L. They are also advised to recheck their glucose every two hours. The “Don’t drive under 5” guidelines are based indirectly on studies linking low glucose to reductions in cognitive function, driving performance and risk of car crash. Prior to our study being published, there was little direct evidence to support these recommendations.

For our study, we monitored the glucose levels of 18 drivers with type 1 diabetes for 3 weeks. All drivers wore a continuous glucose monitoring device. They also used a journey logging device, which automatically recorded their car trips. This enabled us to evaluate the “Don’t drive under 5” recommendations. We asked two key questions.

How many drivers have a hypo while driving, when their starting glucose is above or below 5 mmol/L?

Of the 475 trips by these 18 drivers:

      • 436 trips started with a glucose above 5 mmol/L. There were no hypos (<3.9 mmol/L) during any of these trips.
      • 39 trips started with a glucose below 5 mmol/L. During 10 (26%) of these trips, there was a hypo (<3.9 mmol/L).

Is 2 hours too long to wait until rechecking when driving?

To answer this, we analysed how much glucose changed over time while drivers were behind the wheel. We placed emphasis on decreases of ≥2 mmol/L, relative to the start of the trip. In the figure below, a trend is first noticeable within 20 minutes. A decrease in glucose (≥2 mmol/L) was observed at least once among five drivers.

While high blood glucose is not mentioned in the guidelines, research has indicated that it can impair cognition and could impact driving performance. Therefore, we also analysed increases in glucose during the car trip. We placed emphasis on increases of ≥5 mmol/L, relative to the start of the trip. After 50 minutes, 1 in 3 drivers had an increase of ≥5 mmol/L.

Our study has been published in Diabetes Technology & Therapeutics. It is the first objective evidence in support of Australian recommendations that drivers with type 1 diabetes need to start their trips with a glucose above 5 mmol/L. Our data suggest that waiting 2 hours to recheck glucose may be too long. This is because some people will experience relevant decreases in glucose within 20 minutes and relevant increases within 50 minutes.

Our data are limited by the amount of glucose variation over relatively short trip durations. However, they highlight the need for real-time glucose data and alerts when driving. Future work needs to explore how this critical data for health and road safety can be provided in a format that can ensure minimal driver distraction.


Reference: Trawley S, Stephens AN, McAuley SA, Speight J, Hendrieckx C, Vogrin S, … O’Neal DN for the Australian JDRF Closed-Loop Research Group (2022). Driving with type 1 diabetes: real-world evidence to support starting glucose level and frequency of monitoring during journeys. Diabetes Technology & Therapeutics24(5), 350-356.

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