Stopping Distances and the Unsafe Crossing

09/10/2010 -- In a previous post here, we noted the following information with respect to the stopping distances of various kinds of vehicles:

. . .
Even under the best of roadway conditions (dry pavement, full daylight) and with good brakes, and good reaction time on the part of the driver, an average car traveling at 35 mph needs approximately 100 feet to come to a stop.

This federal standards chart, which became effective just at the end of April, 2010, is the most recent federal standard for braking distance requirements for three axle trucks, which comprise the vast majority of the truck fleet on the highway today. Therefore, it is the federal standard for stopping distances in feet that relate to the typical three-axle trucks one would likely encounter on the roadway today. PFC stands for "peak friction coefficient" and the stopping distances indicated are from the actual application of the brakes until the vehicle comes to a complete stop. They do not include the reaction time of the driver.

Thus, in the best of dry road conditions, a truck traveling at the allowed speed limit of 35 mph, would need a minimum of just under 100 feet to come to a stop once the brakes are applied. One traveling at 40 mph, would need a minimum of 125 feet to come to a stop once the brakes are applied.
. . . .

Cars:

This website contains a simple and useful formula for calculating the approximate stopping distances of cars (but not trucks). Obviously, cars differ from one type of vehicle to another depending on a number of factors, including the kind of brakes (e.g. four-wheel disc versus standard), the condition of the brakes, the reaction time of the driver, etc. To calculate the total stopping distances of an average car at the speeds likely to be encountered at the intersection of Route 165 and Swan Street --35, 40 or 45 mph -- just insert the speed in with "x" representing the speed of the vehicle. Here is that simple formula -- x² ÷ 20 + x = stopping distance. It includes the driver reaction time, plus the actual braking distance once the breaks are applied.

If the speed of the vehicle is 35 mph, 35² equals 1,225; divided by 20 equals 61.25; plus 35 equals 96.25 feet.

If the speed of the vehicle is 40 mph, 40² equals 1,600; divided by 20 equals 80; plus 40 equals 120 feet.

If the speed of the vehicle is 45 mph, 45² equals 2,025; divided by 20 equals 101.25; plus 45 equals 146.25 feet.


Trucks:

Recently, in response to a request I had made for information, I also received a call from an official with the National Highway Traffic Safety Administration (NHTSA), who provided me with additional information to clarify the linked chart regarding truck braking distances.

Though the speed limit in the stretch of Route 165 as it is approaching Swan street is 35 MPH, we all know that many vehicles -- whether cars or trucks --are much more likely to be traveling at 40, or in some cases at 45 MPH. So, based on the information in the chart I've calculated the optimal stopping distances for trucks likely to be encountered on that stretch of the roadway.

The first class of such vehicles would be those identified in the chart as "loaded single unit trucks" which would include trucks like UPS delivery trucks, garbage trucks or perhaps a dump truck.

As can be seen from the chart the manufactured standard for stopping distances for such trucks from the time the brakes are pressed to the time the vehicle comes to a complete stops, are as follows at those three speeds -- 35 mph - 106 feet; 40 mph - 138 feet; and, 45 mph - 175 feet.

We then add on to them the varying distances such trucks would travel before the brakes are engaged -- or, an accepted standard reaction time of operators used by "accident reconstructionists" of 1.5 seconds, before they engaged the brakes.

While that time may be longer than would take many operators to react and hit the brake, experts (such as the one linked above) suggest that more lengthy reaction times occur in situations where, as here, a driver encounters an unexpected surprise -- i.e., a pedestrian suddenly stepping into the roadway.

And the liklihood of that occurring is necessitated because the placement of the bulky relay box has blocked both the vision of the pedestrian (to see what may be coming) and of the driver who, until the pedestrian steps into the crosswalk on the roadway, simply has no idea that there is a pedestrian there!

So the combination represents the distance such trucks could travel before coming to a complete halt. Some drivers might react more quickly, which might reduce the distance a bit, but never likely below 1/3 a second, hence the estimated stopping distances for such trucks in the second chart.

Two Chart Range:
Possible Breaking Distance Ranges for "loaded single unit trucks"

Speed Braking Dist. + Reaction Time 1.5 Sec. = Total Braking Dist.
35 MPH 106 feet + 77.99 feet = 182.99 feet
40 MPH 138 feet + 87.99 feet = 225.99 feet
45 MPH 175 feet + 99.00 feet = 274.00 feet

Speed Braking Dist. + Reaction Time 1/3 Sec. = Total Braking Dist.
35 MPH 106 feet + 17.11 feet = 123.11 feet
40 MPH 138 feet + 19.56 feet = 157.56 feet
45 MPH 175 feet + 22.00 feet = 197.00 feet

Note also that both charts are calculated assuming optimal roadway conditions, such a dry pavement.

Any deviation from optimal conditions, of course, would add to the distance. With wet or icy pavement, the distance might be substantially increased accordingly.

And other circumstances can also effect the reaction time of vehicle operators, such as any distraction that tends to otherwise draw the driver's attention -- turning or passing vehicles, or vehicles otherwise operating at different speeds in adjacent lanes, checking rear-view or side mirrors, distractions such as talking on cell phones, eating or drinking beverages, conversing with passengers, and, of course, driving while fatigued, or while under the influence of drugs or alcohol.

Since a reasonable estimate cannot automatically assume the best of all circumstances, the range between the two sets of distances probably represents the total stopping distances for the vast majority of such vehicles at that intersection, without taking into account those additional factors (poor weather, distractions, fatigue, etc.).

Clearly then, leaving that traffic light as currently configured, is an unnecessary invitation to a tragic outcome. We have already had a few pedestrian accidents involving those crossing Rt. 165 in the recent past.

Even a modest correction, such as the temporary one we have suggested above through the installation of a line of clearly visible ground mounted vertical delineators to "square off" the corner, would 1) of necessity reduce the speed of turning vehicles, 2) allow pedestrians to get a look to see what is coming up Rt. 165, 3) actually shorten the "on road" distance pedestrians travel in crossing Route 165, and 4) give both vehicles and pedestrians emerging from South Franklin Street on to Swan Street, greater visibility and reaction time to avoid impacts with such turning vehicles.

Here is a "overview" sketch drawn on a Google Earth photo of that intersection, and a visible view of how that would work to improve safety in those four ways..

The original overview is courtesy of ling time Lambertville resident and traffic design expert, Gary Toth.

The green line sketch suggesting where the ground-mounted verticle delineators could be installed was added by me. If you "click" on the overview photo, it will expand to a readily viewable size.

UPCOMING RESPONSE: The NJDOT responds!