A Is for Asphalt: A Primer on Modern RoadbuildingWe
rely on roads to accomplish virtually every task we undertake, yet most
of us have only the vaguest notion of how they are actually built. Even
the briefest inquiry quickly reveals that the process involves much
more than a few big machines and a truckload of blacktop—indeed, modern
roadbuilding of the sort that will occur along the Northwest Corridor
is the culmination of an extensive history, sophisticated design, and
advanced technology.
Segment of the Via Appia, an ancient Roman Highway in Italy. (©1976 Kalervo Koskimies. Used with permission.) | | 
Reconstructed Inca suspension bridge, Peru. (©2002 www.clipart.com. Used with permission) | A 2,000 Year Tradition
Until the modern area, the development of roads in human history was
rather uneven. More than two millennia ago, from about 300 B.C., the
ancient Romans were master architects of roads; at its peak, the Roman
system included over 53,000 miles of roads, including 29 separate roads
throughout the Mediterranean, the Middle East, and Great Britain. The
concept of a “highway” actually originated with the Romans, who
elevated their roads two or more feet above the level of the land in
order to minimize the risk of ambush. Originally built to move the
military quickly and efficiently and to speed the collection of taxes,
the Roman road system also brought great trade advantages and forged
connections between remote regions of the Empire.
In many respects, the basic elements of Roman road design remain part
of the modern approach. Roman roads were constructed of several
different layers, increasing in strength from the bottom, with
materials varying according to local availability. The lowest layer was
typically rubble, followed by an intermediate layer of larger stones
cemented with lime mortar, then a layer of brick and pottery fragments
mixed with clay. Finally, the topmost paving layer consisted of flag or
lime grouted stone slabs. Roads were crowned at the center, allowing
drainage to ditches along the roadside. The standards set by the Romans
in terms of durability far exceeded anything achieved after the fall of
the empire; indeed, many of these ancient roads are still major
thoroughfares for cars today, with only the addition of surface
repaving.
After the fall of the Roman Empire, systematic road building ceased in
Europe for almost 2,000 years. By the Middle Ages, roadways typically
were mere dirt paths, and travel was a treacherous and time-consuming
undertaking. Elsewhere during this time, however, the Inca Empire
developed a road network that rivaled the Romans’ in terms of
engineering and scale. Between the 12th and 16th centuries, the Incas
constructed an extensive system of well-built and maintained roads,
covering at least 12,500 miles and stretching along almost the entire
length of the South American Pacific Coast. To cross the many steep
ravines found in the Andes, Inca engineers designed impressive
suspension bridges of rope. Because the Inca had not discovered the
wheel, all travel was done on foot, accompanied by pack-bearing llamas.Blazing the Trail: Early US Roadbuilding
When European settlers arrived in North America, they relied upon bison
paths and the existing network of Indian trails, which connected water
sources and followed mountain passes and short cuts around obstacles.
The first highway established in the United States dates from 1673, and
consisted of the mail route between New York and Boston; soon, post
roads connected the major cities of the thirteen colonies.
The government did not fund road construction after the Revolutionary
War, so private enterprise took on the task of building wagon trails
and “turn pikes” (so called because of the imposing pikes used to build
tollbooth gates). As the 19th century progressed, interest in
roadbuilding temporarily declined, as travel by water continued to be
cheaper and faster. During the 1800s, America’s westward expansion
resulted in the development of several main pioneer trails, followed by
the establishment of several important stagecoach routes. In the latter
half of the 19th century, the railroad came to dominate domestic land
travel in the United States.
As a result of the bicycle craze of the 1890s, bicycling groups began
to agitate for better and more paved roads. During the 19th century,
the macadamized road, named after its Scottish inventor, John Loudon
MacAdam, changed roadbuilding all over the world. Macadamizing involved
a series of layers of successively smaller stones, toped with a top
layer of crushed stone and sand. The materials were sometimes bound
together with a tar-like substance called bitumen, hence the term
“tarmacadam, or tarmac.”
These new surfaced roads proved of great benefit to farmers, whose
horses moved with much less effort than on a dirt road, reducing
transportation time and costs and increasing productivity. A
macadamized road drained well and never turned to mud, as steel
wagon-wheel rims and horseshoes pounded the top layer into a hard,
compacted surface that stayed in place.
The Advent of the Automobile
At the turn of the 20th century, the automobile was still a novelty and
subject to a fair amount of public resistance. In 1900, only 8,000
automobiles traveled US roads—and the roads they encountered were
something less than ideal, particularly in rural areas. Indeed, roads
had become largely obsolete for anything other than short-distance
travel, as people relied on passenger trains for travel of any
distance. According to the first complete survey of America’s roads,
completed in 1904, of the more than two million miles of rural public
roads, fewer than 154,000 miles were “surfaced,” usually with gravel,
stones, or crude paving materials. 
Minneasota Highway Construction, 1910. (MN Historical Society. Used with permission.) |
By 1916, however, the number of automobiles had increased to 2.5
million, and even the best-paved roads were beginning to disintegrate
under their speed and weight. Macadamized roads, which only a decade or
two earlier had seemed so revolutionary, proved unable to withstand the
pounding of constant automobile traffic. The federal government was
slow to respond to the demands for better roads, leading to a great
deal of regional experimentation in roadbuilding techniques.
Authorities experimented with various types of locally-available paving
material—Maryland tried incorporating oyster shells, while some
Southern states attempted to bind cotton into their roads. Whatever its
form, this early roadbuilding was back-breaking, labor-intensive work.
Cement and asphalt soon emerged as the most popular road-paving
surfaces. In 1909, in Wayne County, Michigan, the local authorities ran
an experiment to test brick, granite, wood block, and cement paving
surfaces; cement proved to be the most durable substance by far. As
gasoline consumption increased, the availability of asphalt—a byproduct
of the distillation of kerosene from petroleum—simultaneously
increased, and proved less expensive than concrete.
As the automobile began to dominate domestic travel, public demand for
smoother, elevated, well-drained highways continued to mount. As roads
improved, automobile manufacturers were able to replace early,
high-riding models with lower, and far safer, vehicles. The
accompanying increase in numbers of drivers and maximum speeds,
however, negated the beneficial impact of safer cars—during the 1920s
alone, the motor vehicle death rate rose from 11.7 per 100,000 to 25.7
per 100,000. (By contrast, the death rate for 1996 was 16.5 per
100,000.)
Meanwhile, efforts to build safer, stronger, more durable roads
continued. The Depression saw a great deal of road research, as the
limited funding for highway construction meant that money could not be
wasted on poorly-planned and built highways. If built too quickly, or
with insufficient subbases, roads would wash away in heavy rains,
become riddled with potholes when temperatures fell below freezing, or
simply crumble under the weight of heavy trucks. Thickened-edge
concrete slab paving became the standard in highway construction until
the 1950s. Many of these highways are still in service, and they are
identifiable by the sloping curbs and the rhythmic “thump, thump,
thump” sound as tires cross each joint.
County Road 81: A New Generation of Roadbuilding
Highway engineering has become ever more sophisticated over the last
few decades, as the vehicle speed and the volume of drivers has
increased, and as trucks have become heavier and heavier. A close look
at the modern highway reveals sophisticated engineering, including a
crown that allows water to drain into a complex system of storm drains
and ditches; careful grading; super-elevated, or banked, curves to
permit vehicles to follow the road at high speeds; and wide shoulders
to reduce the chance of injury should a vehicle leave the roadway. 
A steel drum roller smoothing newly laid asphalt. (©2002 Virginia Dept. of Transportation. Used with permission.) |
According to Hennepin County’s Transportation Director, Jim Grube, the
design and reconstruction of the Northwest Corridor will reflect many
of these roadbuilding advances. Hennepin County’s Transportation
Department is responsible for the planning, design, construction, and
maintenance of the County’s highway system, which consists of 567
center-line miles of road, or nearly 1800 miles of traffic lanes. A
typical Hennepin County highway lasts about 40-50 years, meaning that
the County rebuilds approximately 5 to 10 center-line miles of road
each year. Because the soil in the Northwest Corridor is sandier than
in other parts of the County, County Road 81 has held up quite well,
but according to Mr. Grube, the road is due for the reconstruction
planned to begin in 2004.
Planning With Care
As Mr. Grube describes it, rebuilding County Road 81 entails a complex,
multi-faceted process that begins, naturally, with an involved planning
phase. The County first engages in traffic forecasting studies that
look twenty years into the future. This projection helps the County
determine how many lanes are needed, how large the intersections must
be, how thick the blacktop must be, and where the traffic signals will
be located.
The County also must determine the physical features of the road,
including alignment changes and the geometry of curves. The route will
largely follow the existing alignment, although the addition of busway
lanes will result in a widening of the road. Because the existing road
is quite flat, Mr. Grube does not anticipate the need for many
elevation or grading changes. One exception is the bridge over the
railroad north of Hwy 100 in Crystal, which will have a more gradual
approach following reconstruction. Intersections along the corridor
will also be made wider to accommodate the anticipated heavier flow of
traffic.
The County must ensure that the project complies with all state
regulations. The state requires that the County study old Pollution
Control Agency files to determine former land uses, including the
location of old gas stations, and perform any needed clean-up. The
County must determine whether the roadway is adjacent to areas
harboring protected animal populations, such as spawning fish, nesting
birds, or certain species of turtles or frogs; if so, appropriate
precautions must be taken. Finally, the County must conduct an
assessment of potential noise and air pollution impacts twenty years
into the future, based on the current land use zoning adjacent to the
roadway.
Although Hennepin County’s engineers and planning specialists typically
perform all facets of this planning work, the County has retained a
consulting firm to handle the design phase on this project because of
its accelerated timeline. According to Mr. Grube, the County Road 81
reconstruction is advancing approximately three years ahead of a normal
schedule. The County still retains final responsibility for oversight
of all aspects of the reconstruction.
Once each city has approved the preliminary design, the County will
enter a more advanced and detailed design phase. One facet of this work
is an assessment of the soil structure in the ditches adjacent to the
roadway. Because the roadbed will be expanded to accommodate the
busway, the soil in the ditches must be corrected to support the load.
During this phase, the County will continue to hold regular open houses
and remain in continual communication with landowners affected by the
project, whether by an encroachment or because of reduced access due to
the construction. Jim Grube describes this as one of the most critical
aspects of the planning phase, since all of the property purchase or
easements must be in place before crews can begin work. Finally, at the
end of the design phase, the county will publish the design and
advertise for bids from the large roadbuilders.
Once a company has been chosen and the County has purchased the
necessary land or acquired easements, construction will begin. The
first phase involves demolition of the existing road. Large milling
machines with conveyer belts will grind up the blacktop pavement and
transfer it into dump trucks. The blacktop will be hauled away to
facilities to be crushed and recycled into new roadbuilding material.
After the old pavement is gone, large backhoes will begin to remove the
top soil from the ditches in areas where the roadbed will be expanded.
Sets of trucks, working in tandem, will haul the soil away and replace
it with loads of sand.
The Proper Materials
The road itself will be made up of three layers, a subgrade of sand, a
subbase of gravel, and a paving surface of asphalt cement concrete,
otherwise known as bituminous pavement or black top. Blacktop is a
combination of sand, gravel, and the oils that bind them together.
Another common paving substance, Portland cement concrete, will be used
for the curb and gutter and the bridge over the railroad in Crystal.
High-volume roads are typically built entirely of this white Portland
cement; by contrast, asphalt cement is the more common paving surface
for roads such as County Road 81, which do not bear the heavy volume of
truck traffic as interstates and major US highways.
In order to ensure adequate drainage, the County builds a typical
highway with a subbase of about two feet of sand, topped with 4-5
inches of gravel and 4-7 inches of blacktop. “Because of the
temperature and moisture in our climate,” explains Grube, ”it is
critical that we use the proper materials in the proper proportion. We
run into trouble when rainwater does not drain away quickly enough
before a freeze, because the pressure from the expanding frozen water
causes the road to heave, leading to potholes.”
After the sand is in place, backhoes will place the underground
stormsewer piping and dig new stormwater treatment ponds. As with all
new road construction, the County will replace the existing system of
drainage ditches with an underground storm sewer that pipes stormwater
into treatment ponds. Once the drainage system is in place, trucks will
begin hauling in gravel so that the crews can build the actual grade of
the newly-expanded road. Next, cement trucks will appear, as crews
begin building curbs and gutters. The actual construction phase will
typically take about two years, since crews can usually work only from
mid-April until Thanksgiving in our climate.
Bells & Whistles
The finishing step, of course, will be painting traffic lines and
installing traffic signals. The traffic signal system along County Road
81 will make use of Intelligent Transportation Systems (ITS) to improve
traffic flow and to accomplish specific goals, such as affording buses
time advantages at intersections. As Jim Grube explains it, “ITS
technology will allow the signal system to be set so that if a light is
green and a bus is approaching, the light will remain green long enough
for the bus to pass through. Similarly, if the bus is waiting at the
intersection, its light may turn green before the other lanes so that
it can move ahead sooner.” ITS technology also enables traffic signals
to read special strobe lights from approaching emergency or police
vehicles, move into Emergency Vehicle preemption mode, and modify
signals to speed the passage of the vehicle through the intersection.
Another example of ITS that may well be present along County Road 81 is
the use of automatic vehicle location devices on buses. Using these
devices, buses send signals to overhead satellites, which feed the
information about bus location into display terminals at the bus stops
so that people waiting for buses know exactly when the bus is due to
arrive. 
Example of a milling machine, used to grind up old asphalt pavement for recycling. | The
impact of these bells and whistles is impressive, but these
technological advances are not the most significant development in
roadbuilding that Jim Grube has observed over the last few decades.
“The equipment has become bigger, and the electronic controls are much
finer and more sophisticated, but the most dramatic change that I have
seen in my years in this industry has been the widespread adoption of
recycling roadbuilding materials,” says Grube. “When I started working
in this business, ripped-up blacktop from the City of Minneapolis was
used to fill ‘swamps’ in the first ring suburbs, so that the cities
could build parks on that land. Now, we recognize those ‘swamps’ as
protected wetlands, and that ‘waste’ blacktop is reused to build more
roads. We have an entirely different awareness of our responsibility to
minimize the impact of roadbuilding on our environment.”
Sources
Kaszynski, William. The American Highway, McFarland & Co: Jefferson, NC, and London. (2000).
Stilgoe, John. Roads, Highways, and Ecosystems. The Use of Land:
Perspectives on Stewardship, available on National Humanities Center
web site at: www.nhc.rtp.nc.us:8080/tserve/nattrans/ntuseland/essays/
roads.htm
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A Look Back at Robbinsdale A Is for Asphalt: A Primer on Modern Roadbuilding From Farm to Market: The Changing Landscape of Brooklyn Park Robbins and Parker: Pioneering Partners in the Northwest Corridor Building Communities: The Beginning Years of the Northwest Corridor Pierre Bottineau: Frontiersman of the Northwest Corridor and Beyond
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