Stopping Sight Distance

Here's a lesson that will help you on the afternoon session if you plan on taking the civil-specific test. I don't think there are enough discipline-specific afternoon study materials out there on the web for free, so I'm going to make some, starting with Stopping Sight Distance (SSD).

You don't need to know this to pass the exam, but to refresh your memory, SSD is basically the length of roadway it will take a vehicle operating at certain design parameters and assumptions to stop before it hits an object in the middle of the road. It all boils down to time multiplied by velocity to get a distance, but there are two time components. First, how long does it take the driver to react to this object he's about to crash into? Second, how much time does it take the vehicle to come to a complete stop?

This is important to know when designing highways. For example, if you're designing a road with a 50 mph design speed, you'll want to know the stopping sight distance so you can design things like your horizontal and verticle alignment in such a way that drivers don't come over the top of the hill and smash into a stalled truck on the other side.

Page 162 of the NCEES FE Supplied Reference Handbook has the following equation (and defines the variables):

Where:

• S = Stopping Sight Distance (SSD)
• V = Design Speed
• a = Deceleration rate
• G = Highway Grade
• t = Reaction Time

From what I've seen, the FE questions will give you all of this information and more and at this point it's pretty much my favorite type of problem: plug & chug.

The best way to get this problem down is to do a few practice problems so you get acquainted with the variables and recognize the equation when it's time to open up your book. Then on the exam you can just pick apart the problem statement and crank it out. If a SSD problem is on the afternoon test (it won't be on the morning), it's easy double points.

One thing you might memorize is that AASHTO recomments a default value of 11.2 feet per second squared for "a". If it's not given on the exam, I'd plug in 11.2 and see what happens. I've seen on old practice exams where they've asked you to calculate it given friction factors and such, but that formula is no longer in the handbook, so I am going to knock on wood and hope they don't want you to pull it out of your you-know-where. If you want to be thorough, it's out there.

One last tip: watch the units. Make sure the problem statement's units match up with the variables on Page 162 of the handbook. This formula is an oddball in that the units shouldn't be consistent. If you're sharp you'll notice that the conversion factors are "built in". If the units for the information given don't align with what's listed on Page 162, convert them. I assume that by now you know how to do basic unit conversions and that there are, for example, 5280 feet in a mile and 3600 seconds in an hour (unfortunately this is one subject area where we are still stuck in US customary units and the exam hasn't gone to SI).

Practice Problems:

Problem 1) Find the Stopping Sight Distance of the following roadway:

• Design speed = 40 mph
• Driver reaction time = 2 seconds
• Grade = 0%
• Decelleration rate = 11.2 ft/s^2

Problem 2) A driver with a 2.5-second reaction time is travelling at 65 miles per hour up a 2% grade. Assuming AASHTO standard values for all other information, what distance will be required for the driver to come to a complete stop if an obstruction enters his or her field of vision?


Solutions: Highlight the lines below with your mouse to view the answers, or click the links to view a scan of my hand-written solution:

Problem 1) 271 ft worked solution
Problem 2) 622 ft worked solution