Heidi Shaffer, Published November 21 2010
Reading the signs of speeding: Traffic blitz reveals driving tendencies
Speed limits are determined using a number of variables, including in what type of area the road is located and the volume of traffic. Those variables differ depending on who governs a given roadway.
Both North Dakota and Minnesota set statewide speed limits, but individual cities can dictate a posted limit for certain streets, such as main collector roads and arterials.
In North Dakota, residential and other local street speeds are limited to 25 mph unless otherwise signed. In Minnesota, those same types of roadways carry a 30 mph limit.
If a speed differs from the state limit, the city must make sure it’s signed on all entry-points to the roadway, said Moorhead Engineer Tom Trowbridge.
If Moorhead wanted to designate residential areas a 25 mph limit instead of the state’s 30 mph, the signage involved would get very expensive, he said.
Moorhead must also have the Minnesota Department of Transportation conduct a speed study to determine whether a road should have a limit that varies from state law, Trowbridge said.
In Fargo, all roads are 25 mph, unless otherwise posted, said Jeremy Gorden, the city’s traffic engineer.
Fargo speed limits are generally based on what kind of area is surrounding the streets, Gorden said.
If the area is residential, limits tend to be slower; faster speeds are common in commercial zones, he said.
“You don’t want people going 35 in a residential area,” he said.
Typically, collector streets carry 30 to 35 mph limits. Main arterial – as on 45th Street and portions of 32nd Avenue South – limits are usually 40 or 45 mph. For example, Main Avenue speeds go from 30 mph in downtown Fargo to 35 and eventually 45 mph as the road moves through more industrial and commercial areas to the west of downtown.
Limits vs. reality
There is a realization from both engineers and law enforcement that people tend to drive faster than the speed limit.
Engineers usually design roads based on the assumption that traffic will move at about 5 mph over the limit, Gorden said.
Police are also aware of the tendency to drive faster than posted limits.
“Rarely do you see someone driving just the 25,” said Sgt. Mike Bernier of the Fargo police.
“And we know that going into (enforcement),” he said.
Traffic engineers have a few tools to slow down traffic, but police enforcement is often the first step.
Trowbridge said he often gets requests from Moorhead residents wanting stop signs to slow down neighborhood traffic.
But stop and yield signs are actually intended to assign right-of-way on busier roads, not slow traffic down, Trowbridge said.
Fargo will use roundabouts along a new stretch of 25th Street South near Davies High School to calm traffic on what will be a wide-open stretch of road, Gorden said.
The speed limit will be 35 mph, but the intersection roundabouts will slow traffic to about 20 mph, he said.
Fargo homeowner Shane Gebeke has battled the city for years on speeding traffic along 20th Avenue South, which runs in front of his home.
Drivers often take 20th Avenue from University Drive to 25th Street, and motorists don’t obey the 25 mph signs posted, Gebeke said.
Gebeke said the effects of enforcement are often short-lived and wants the city to include on-road fixtures, such as speed tables.
“Twenty-five mph seems fine to me, but nobody does it on that street,” he said.
Setting the speed
Speed limits are usually known when a road is built, but once the street opens, adjustments may be needed, Gorden said.
Fargo speed limit changes go to the city commission or North Dakota Department of Transportation if a street is a state highway. All Moorhead city speed limit changes that differ from state law are approved by MnDOT after a speed study is completed.
Speed limits are also based on what motorists feel comfortable driving on a given road, Gorden said.
The limit on 36th Street South near Mills Fleet Farm was increased from 30 to 35 a few years ago based on the fact that the stretch is fairly wide open and feels more like a highway, Gorden said.
To determine a speed change, traffic engineers study how fast motorists tend to drive on the road and use the speed at which 85 percent of traffic is driving at or below to determine what limit should be posted.
Using that type of survey, engineers found drivers on 32nd Avenue South were actually driving below the posted 40 mph speed limit at which the arterial was marked in 2009. Later that year, the speed limit was changed to 35 mph.
The 32nd Avenue South study was based on police requests to look into the thoroughfare because of a number of serious accidents that were occurring.
Accidents tend to become severe on roads where speeds are higher than 40 to 45 mph, Gorden said.
Police requests and an increase in accidents result in engineers re-examining limits, he said.
Sgt. Bernier said police often get calls from residents concerned about speeds being both too fast and too slow. But as for now, the limits are appropriate and fair for most areas of the city, he said.
Readers can reach Forum reporter Heidi Shaffer at (701) 241-5511