  This paper 1) analyzes the extent to which drivers engage in multitasking
additional-to-driving (MAD) under various conditions, 2) specifies odds ratios
(ORs) of crashing associated with MAD compared to no task engagement, and 3)
explores the structure of MAD, based on data from the Second Strategic Highway
Research Program Naturalistic Driving Study (SHRP2 NDS). Sensitivity analysis
in which secondary tasks were re-defined by grouping similar tasks was
performed to investigate the extent to which ORs are affected by the specific
task definitions in SHRP2. A novel visual representation of multitasking was
developed to show which secondary tasks co-occur frequently and which ones do
not. MAD occurs in 11% of control driving segments, 22% of crashes and
near-crashes (CNC), 26% of Level 1-3 crashes and 39% of rear-end striking
crashes, and 9%, 16%, 17% and 28% respectively for the same event types if MAD
is defined in terms of general task groups. The most common co-occurrences of
secondary tasks vary substantially among event types; for example, 'Passenger
in adjacent seat - interaction' and 'Other non-specific internal eye glance'
tend to co-occur in CNC but tend not to co-occur in control driving segments.
The odds ratios of MAD compared to driving without any secondary task and the
corresponding 95% confidence intervals are 2.38 (2.17-2.61) for CNC, 3.72
(3.11-4.45) for Level 1-3 crashes and 8.48 (5.11-14.07) for rear-end striking
crashes. The corresponding ORs using general task groups to define MAD are
slightly lower at 2.00 (1.80-2.21) for CNC, 3.03 (2.48-3.69) for Level 1-3
crashes and 6.94 (4.04-11.94) for rear-end striking crashes. The results
confirm that independently of whether secondary tasks are defined according to
SHRP2 or general task groups, the reduction of driving performance from MAD
observed in simulator studies is manifested in real-world crashes as well.
