The effects of on-street parking and road environment visual complexity on travel speed and reaction time

  • Published: Accident Analysis and Prevention, 45, 759-765, 2012
  • Authors: J. Edquist, C. M. Rudin-Brown, M. G. Lenné
  • Date Added: 18 Apr 2013
  • Last Update: 18 Apr 2013
  • Format: pdf


Examine differences in driver behaviour which may help to explain RTI risk differences between different environments


Driving simulator study examining the effects of on-street parking and road environment visual complexity on driver behaviour and surrogate measures of RTI risk. Twenty-nine participants drove a simulated urban commercial and arterial route.

Key Findings:

  • On-street parking is also associated with increased RTI risk compared to roads of the same category without on-street parking.

  • One potential reason for increased RTI risk is narrowed road width. Parked vehicles leave less space for travelling vehicles, forcing them to drive closer to vehicles in the next lane (which may be travelling in the opposite direction). Similarly, narrow roads are associated with shifts in lateral position closer to the centreline as well as higher RTI rates than standard roads.

  • Modelling research found that both parking and road width were significant predictors of RTI rates on road links, and that the contribution of parking to increased RTI rates was larger than that of road width. This implies that factors other than road width must underlie the higher RTI risk when on-street parking is present.

  • In complex urban environments, drivers must monitor movements of both pedestrians and vehicles. Parked cars may obstruct the view of the road ahead, making it more difficult to see crossing pedestrians.

  • Parked cars may also suddenly become moving cars, and rejoin the traffic stream. It has been found that the number of (moving) vehicles in a scene negatively affects situation awareness and hazard avoidance, and parked vehicles may contribute to this effect, as they require monitoring to determine whether or not they are moving. The presence of parked cars therefore increases the uncertainty, mental load and potential risk associated with the road environment.

  • Traffic observations in residential areas have found that high parking densities correlate with slower speeds.

  • Compared to sections with no parking bays or empty parking bays, in the presence of occupied parking bays drivers lowered their speed and shifted their lateral position towards roadway centre to compensate for the higher mental workload they reported experiencing. However, this compensation was not sufficient to reduce drivers’ reaction time on a safety-relevant peripheral detection task or to an unexpected pedestrian hazard.

  • Compared to the urban road environments, the less visually complex arterial road environment was associated with speeds that were closer to the posted limit, lower speed variability and lower workload ratings. These results support theoretical positions that proffer workload as a mediating variable of speed choice. However, drivers in this study did not modify their speed sufficiently to maintain safe hazard response times in complex environments with on-street parking. This inadequate speed compensation is likely to affect real world RTI risk.


On-street parking, RTI risk, Driving simulation


Useful research but was conducted inAustralia where road environment might be different to the UK.