The Science of a Two Vehicle Scenario

The science of two car scenarios

The ISDA 2015 and preliminary data from the 2017 Executive/Security Vehicle Survey indicate a dramatic increase in the use of SUVs for Executive Transportation. The 2015 Survey also indicated that there had been a significant increase in the use of two car scenarios for Secure Transportation. If you couple the Survey data with the number of motorcade incidents that have occurred it points to a problem in the making.

The following is an explanation of why a two car scenario can create a risk to the principal and some suggestions on how to mitigate that risk. The risk starts with what scientists call “driver’s eye height.”

The Driver’s Eye Height

A study was conducted to assess drivers’ chosen speeds when operating a simulated vehicle, doing so while viewing the road from a low eye height (if a sedan would be the lead vehicle) and a high eye height (if a SUV would be the follow vehicle). Participants were instructed to drive, without reference to a speedometer, at a highway driving speed at which they felt comfortable and safe. Drivers seated at a high eye height (the follow vehicle) drove faster than when they were seated at a low eye height (the lead vehicle).

Consider that the majority of follow vehicles are SUV’s and that the seat in a typical SUV is about 1.6 feet higher than in a car. Apply this theory to a two car scenario where the principal is in a sedan, (the driver sitting at normal eye height) and the follow car is an SUV with the driver sitting at an elevated eye height. If both drivers perceive that they are moving at the same speed – for example, they both feel they are driving at 40 MPH – due to the eye height of the driver in the follow car he/she will be driving faster than the driver in the principal car. According to the study, the follow car’s speed can be off by as much as 15%. So if the driver in the follow car thinks they are driving 40 MPH, they are more than likely driving 46 MPH. If the driver in the follow car thinks they are driving at 60 MPH, they are probably going 69 MPH. And the higher you sit in the vehicle, the higher the speed differential.

To carry this one step further, an increase in speed of 15% would represent an increase of 30% in the energy the driver will need to manage in an emergency maneuver and an increase in 30% in the distance required to stop in the event of an emergency.

All this leads to the fact that driving the follow car is not easy; following distance will vary and require the driver’s constant attention.

Some thoughts on the two vehicle scenario:

  1. The driver of the follow vehicle needs to be cautious of their speed. Lots of consideration needs to be given to who is driving the vehicle; putting a driver with little or no experience driving an SUV in a motorcade is problematic.
  2. Preliminary data from the 2017 Executive/Security Vehicle Survey also indicates that when traveling the most often rented vehicle is an SUV, driven by a subcontracted security driver. We suggest that the driver has experience driving SUV’s, and is aware of the misconception of speed that an SUV creates.
  3. There is a substantial difference between the handling capability of the lead vehicle (if a sedan), and the handling capability of the follow vehicle (if an SUV). The difference in handling capability, when coupled with the misconception of speed created by the follow driver’s eye height, can create a significant risk. The driver of the lead vehicle must be aware of their cornering speed. They need to understand that the follow car may not be able to corner at the same speed as the lead vehicle. They need to coordinate their movements. This can be accomplished by advancing the route.
  4. Be cautious of the difference in stopping distance between a sedan and an SUV. Scenario: the lead vehicle is a Mercedes S 550 which can stop from 60 MPH in 128 feet. The follow-vehicle is a Suburban LTZ which can stop from 60 MPH in 138 feet. The Mercedes comes to a quick stop. At the elevated eye height, the Suburban driver could be moving at 69 MPH and take 30% more distance to stop the vehicle, so not accounting for reaction time the Suburban would need about 179 feet to stop the vehicle. If the driver of the follow vehicle is reacting to the brake lights of the lead vehicle, and considering an average reaction time of .75 Seconds, there would be an additional 76 feet added to the stopping distance, which would make the total stopping distance 255 feet – which would present a problem.

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