How Design Speeds Dictate Posted Speed Limits
Dozens of factors combine to determine posted speed limits on highways and local roads. Among those factors are vehicle limitations, weather conditions, non-motorized road users, old ladies writing letters to city council, and — perhaps most fundamentally — design speed.
What is design speed? Read on, my friends.
Disclaimer: This article is for entertainment purposes only. Always obey local traffic laws, including posted speed limits, always wear your seatbelt, don’t drink and drive, don’t text and drive, yield to pedestrians, and always dispose of used motor oil properly.
According to the Federal Highway Administration (FHWA), design speed as a concept was first introduced in 1936.Its first definition described design speed as “themaximum reasonably uniform speed which would be adopted by the faster driving group of vehicle operations, once clear of urban areas.” More recently,FHWA changed the definition toreflect the highest sustained speed permitted by the features of the roadway itself under ideal conditions. Design speeds generally do not change after a roadway is first built or is substantially rebuilt because its underlying topography dictates it.
A road’s design speed are determined by a number of factors: sight distance, vertical curvature, horizontal curvature, and superelevation (banking).
One of the most basic necessities for driving at any speed is sight. Drivers must see a sufficient distance ahead in order to avoid an obstacle or a crash. The American Association of State Highway and Transportation Officials (AAHSTO) published a table of recommended stopping sight lines for various road speeds. In the table, minimum sight distances are recommended for various speeds. A road designed for 25 mile-per-hour traffic should have at least 155 feet of stopping sight distance, a highway designed for 70 mph traffic should have 730 feet of stopping sight distance. These guidelines aren’t mandatory, but are widely accepted.
While stopping sight distance is easy to visualize, vertical curvature is slightly more abstract. Vertical curvature measures the change in slope steepness. It occurs at the tops and bottoms of hills or valleys.
In the picture above, we can see that the sight line drawn from the following car to the leading car is at risk of being cut off by the hill’s crest. A crest like this would limit vehicle speeds by temporarily reducing stopping sight distance compared to a flat road or one with constant slope.
At the bottom of a decent, the effect of sag is felt mostly at night. On an unlit road, the vehicle’s headlights may not completely illuminate the necessary stopping sight distance. Instead, the vehicle’s headlights point at the bottom of the valley instead of downrange. This condition would not be as noticeable during daylight or on a road equipped with street lighting.
And now we break out the slide rulers and pocket protectors.
Horizontal curvature, or turns in normal-speak, are a major factor in determining design speeds. The AASHTO guidelines use a scary looking math equation to decide design speeds on curved road segments based on the amount of allowable “side friction,” or — more plainly — lateral Gs.
The equation basically determines cornering speeds based on maximum lateral G loads. The equation changes when superelevation, or banking, is factored in. On a super-elevated turn like a cloverleaf on-ramp, some of the cornering forces would press the vehicle into the pavement instead of being fully counted as lateral Gs.
However, much of this hot Ti-83 action is unrelated to a vehicle’s ability to handle curves without losing control. Instead, it’s passenger comfort that dictates the maximum lateral G load. Worse yet, it’s based on passenger comfort tests carried out as early as the 1940s, when neither women nor front suspensions were independent.
From the FHWA:
The values currently in use are based on research of passenger comfort (blindfolded passengers) conducted in the 1940s. Subsequent advancements in vehicle technologies call into question the validity of the historic research and its continued use for design and analysis purposes.
So how then are design speeds reflected in posted speed limits? Well, it’s complicated.
According to a report from Purdue University, speed limits on rural and suburban highways are typically set between zero and 10 mph below the highway segment’s design speed. But it isn’t just design speed that dictates a given speed limit.
Other factors identified in the report for setting speed limits on a given road segment were: 85th percentile free-flow speed, design speed, road surface characteristics and condition, road classification (limited-access highway, two-lane road, etc.), type and density of roadside development (residential area, farmland), roadside or on street parking, 12 month accident history, and pedestrian activity.
[Images: Speed Limit Sign, James Brooks/Flickr; Figures, Federal Highway Administration]
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