Posted on 12/12/2018 12:54:09 PM PST by ETL
Speed limits set only five miles per hour below engineering recommendations produce a statistically significant decrease in total, fatal and injury crashes, and property-damage-only crashes, according to a group of Penn State researchers.
"If (however) you lower the speed limit by 10, 15, 25 miles per hour, or more, drivers stop paying attention," said Vikash Gayah, assistant professor of civil engineering. "We found there was an increase in fatal and injury crashes at locations with posted speed limits set 10 miles per hour or more below engineering recommendations."
Speed limits are normally set based on results from engineering studies that collect free-flow traffic data and then select an appropriate speed using a statistical model. However, factors such as school zones, citizen or political pressure, and perceived safety issues contribute to the fairly common practice of lowering speed limits below engineering guidelines, the researchers report in Accident Analysis and Prevention.
"When doing speed research, we are looking at free-flow speeds—the speed that drivers select based on geometric and prevailing weather conditions," said Gayah.
The team of researchers collected speed data on three different occasions from 12 roadway segments in Montana, a state that posts speed limits lower than engineers advise. Eight of the 12 sites had posted speed limits changed below engineering recommendations either by 5 mph, 10 mph, 15 mph, or 25 mph. The other four sites had posted speed limits set equal to engineering recommendations and served as comparison locations.
Each period of data collection considered either the presence of no law enforcement, light law enforcement, or heavy law enforcement.
Speed data was collected in daylight hours and in fair weather conditions using hidden pavement sensors. Large vehicles, such as trucks, and cars traveling too close together were excluded. Cars traveling less than 10 mph of the posted speed or greater than 20 mph of the posted speed limit, known as speed outliers, were also excluded. Crash history data from a four-year period before and after speed limits changed was also used in the analysis.
The researchers found that vehicles were two times more likely to obey the speed limit at locations with higher posted speed limits set at 50 mph or 55 mph compared to the base case of less than 50 mph, and four times more likely to obey when the posted speed limit was between 60 and 70 mph.
The presence of heavy law enforcement in zones with low posted speed limits showed an average reduced speed of 4 mph and greater speed-limit compliance. "The practice of setting speed limits lower than what would be recommended from an engineering study is okay if it's only by a little—by five miles per hour," said Gayah.
Such a difference showed an increase in speed-limit compliance, a decrease in property damage only, and total, fatal and injury crashes. Larger differences between the posted and engineering-recommended speed-limit values were found to increase crash frequency and reduce speed limit compliance.
Explore further: Drivers go faster than what they think is safe in roadworks zones
Especially when that old lady with the big floppy hat, and the never-ending turn signal is driving slow in the fast lane.
When you lower them too much, you increase the disparity between those who obey the speed limits and those who don’t give a sh!t. The former end up getting run over by the latter.
If you drive twice as fast you will spend half as much time on the road. Less time spent running the risk of having an accident.
HELLOOOOO! Of COURSE they do. Not to mention the ROAD RAGE THEY CAUSE. I call them Hubbites. They have their heads up their butts when they’re driving.
I also think they all wear Depends. Hell, they’re wearing their Depends, they don’t care if they EVER get where they’re going
Makes sense. Good point.
The only why you can rear end someone, is if they are going slower than you.
Not just DUI
Most accidents occur because people are driving HUA
(Head Up Arse)
This standard doesn’t universally apply. There was an infamous S-Curve on Lake Shore Drive near downtown Chicago that had a 15 MPH speed limit ... and it MEANT IT. That really was the engineered speed. Every weekend some car with out of state plates would find itself stuck halfway over a Jersey barrier.
There’s a section of road I drive during my commute that absolutely nobody obeys the limit. No car, truck, bus, ambulance, motorcycle, is travelling at 18 MPH (30 kph) for the 3/4 mile strip.
Okay, that’s an exaggeration. New drivers and people with radar detectors that notice there’s a speed trap there are doing the speed limit.
The city went so far as to install one of those speed boards that flash if you are travelling more than 10 kph (6 MPH) above the posted limit. Every single vehicle going through there gets the flashing lights and frowny face. Every. Single. Vehicle.
Not just DUI
Most accidents occur because people are driving HUA
(Head Up Arse)
Excellent!
Only drive as fast as I can see - Ol’ Jack Burton ...
“Did you ever notice that anyone who drives slower than you is an ‘idiot’, and anyone who drives faster than you is a ‘maniac’?”
-George Carlin
They installed one of those speed signs on hollywood blvd.
Lasted about 2 hours before someone threw a brick through the display.
Seems are locals don’t care for those red light cameras either.
They keep getting nailed with paintballs.
Speed limits signs are only suggestions.
If there is a car on fire in your lane, change lanes.
Reminds me of a friend of my grandmother’s. She was pulled over by the Colorado Highway Patrol on I70 about 50 miles east of Denver, in the left lane, driving about 40 miles an hour. When the trooper asked her what she was doing in the left lane, she told him she needed to turn left when she got to Denver.
The biggest problem in terms of crashes is WIDE deviations between fasted drivers and slowest drivers... Far safer to be on a road where EVERYONE is going 65, than a rode where folks are going anywhere from 40-65 MPH all at the same time.
“When you lower them too much, you increase the disparity between those who obey the speed limits and those who don’t give a sh!t. The former end up getting run over by the latter.”
The thought is good... but if you look at the study details the excluded looking at vehicles traveling significantly over the limit. They ignored those as outlier
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