Nov 3, 2020
The topic of this week's episode is Decision Sight Distance or DSD for short.
Decision Sight Distance plays an essential role for those that provide secure transportation. But understanding DSD is vital for anyone who drives an automobile.
As a quick refresher for those unfamiliar with DSD, the Decision Sight Distance definition is the length of road surface drivers can see and have an acceptable reaction time. In the US, the organization that is responsible for designing our highways, the American Association of State Highway and Transportation Officials (AASHTO), have guidelines concerning "line of sight," and from those guidelines developed the all-important Decision Sight Distance (DSD)
AASHTO defines DSD as the distance needed to recognize a problem and complete a maneuver safely and efficiently. And according to the scientists who have done an enormous amount of research on driver reaction time, the "average" driver needs 2.5 seconds to complete the "recognize a problem" part of the DSD.
We have discussed decision-sight distance in past podcasts and articles, but now we would like to present a DSD visual representation. For our podcast listeners, we’ll do our best to describe the visualization.
We'll use the Omar García Harfuch (OGH) scenario. Keep in mind that we are using the OGH scenario for illustration and demonstration purposes only. The attack occurred in the early morning. All the pictures used for this podcast are from Google Street View in daylight. The significant difference is that the OGH’s driver was looking at headlights crossing the intersection—more than likely assuming that the truck would take a left onto Ave Reforma.
To establish a timeline, we looked at the video of the attack; we found that it took approximately 4 seconds for the truck to pull out across and block the intersection. So, from the first time the driver could have seen the truck's headlights to the time the truck stopped, blocking the intersection was four seconds.
Keep in mind that all vehicle attacks or accidents are a time distance relationship - as the blocking vehicle pulled into the intersection, the question is how much time did OGH’s driver have to react? – the answer depends on the speed of the vehicle. If the driver was moving at:
20 MPH/32.2KPH the driver was 8.7 seconds from the Intersection
40 MPH/64.4 KPH the driver was 4.4 seconds from the Intersection
50 MPH/80 KPH the driver was 3.5 seconds from the Intersection
60 MPH/96.6 KPH the driver was 2.9 seconds from the intersection
To get a sense of how much distance the driver had to work with, we use landmarks on Paseo de la Reforma for reference points. The picture depicts the driver's eye view 100 Meters or 328 Feet away from the intersection. If you look at the top right-hand corner of the picture, you can make out a car stopped at the intersection.
We estimated from our forensic analysis that the vehicle was traveling 65 kilometers per hour (or 40 MPH).
From the 100 Meter mark at 65 KPH per hour 40 MPH, the Suburban vehicle was approximately 5 1/2 seconds from the intersection. They were closing in at the rate of 18 Meters Per Second (MPS) or 58.8 Feet Per Second (FPS).
As they continued down the Paseo de la Reforma moving at 65 KPH per hour 40 MPH, 2.8 seconds later, the vehicle is at the 50 Meter – 164 Foot mark.
The picture shows the driver's eye view at 50 meters – 164 feet from the intersection. You can see the car clearly now; hence the principal vehicle's driver could have also seen what was about to happen.
From the 50 Meter – 164 feet mark one second later, this picture is the driver's view.
They are 32 Meters 105 feet from the intersection. Assuming that they may have decreased their speed. At this location on the road, if traveling at
20 MPH – 32.2 KPH they are 3.5 Seconds away from the intersection
30 MPH – 48.3 KPH they are 2.4 Seconds away from the intersection
40 MPH – 64.4 KPH they are 1.8 Seconds away from the intersection.
Secure Transportation
Sight distance plays a vital role in supplying safe and secure transportation. It is a significant factor in determining if the event you drive into is winnable. During your route survey, know how far you can see and DO NOT DRIVE FASTER THAN YOU CAN SEE – Which means drive at a speed that will give you the time to react at the given sight distance.
As you are conducting a route survey, the question you need to ask yourself is – At the speed I am moving with the given sight distance, how much time do I have, and in that time frame, what can I do with this vehicle? No matter the scenario, accident, or vehicle violence, if you don't have enough sight distance at the speed you are moving, it is a no-win scenario. Your training must take this into account.
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That’s it for this week, I hope you will join us next week for another episode of the EPST podcast. Show notes as well as the short visualization for this episode are available at SecurityDriver.Com website. Make sure to subscribe to the podcast in your favorite podcast app and if you’ve been listening for a while, let us know what you think by leaving us a review.
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