Controlled-access highway

See also: Limited-access road

The cloverleaf interchange between US 131, M-6 and 68th Street in Cutlerville, Michigan, United States, shows many of the features of controlled-access highways: entry and exit ramps, median strips for opposing traffic, no at-grade intersections and no direct access to properties.

This sign, or some variation thereof, is used to denote controlled-access highways in many countries.

Typical overhead signage on Ontario's King's Highway network featuring an airport pictogram, distances to upcoming interchanges, and lane guidance

A controlled-access highway is a type of highway that has been designed for high-speed vehicular traffic, with all traffic flow—ingress and egress—regulated. Common English terms are freeway, motorway and expressway. Other similar terms include throughway and parkway. Some of these may be limited-access highways, although this term can also refer to a class of highways with somewhat less isolation from other traffic.

In countries following the Vienna convention, the motorway qualification implies that walking and parking are forbidden.

A fully controlled-access highway provides an unhindered flow of traffic, with no traffic signals, intersections or property access. They are free of any at-grade crossings with other roads, railways, or pedestrian paths, which are instead carried by overpasses and underpasses. Entrances and exits to the highway are provided at interchanges by slip roads (ramps), which allow for speed changes between the highway and arterials and collector roads. On the controlled-access highway, opposing directions of travel are generally separated by a median strip or central reservation containing a traffic barrier or grass. Elimination of conflicts with other directions of traffic dramatically improves safety, while increasing traffic capacity and speed.

Controlled-access highways evolved during the first half of the 20th century. Italy opened its first autostrada in 1924, A8, connecting Milan to Varese. Germany began to build its first controlled-access autobahn without speed limits (30 kilometers [19 mi] on what is now A555, then referred to as a dual highway) in 1932 between Cologne and Bonn. It then rapidly constructed a nationwide system of such roads. The first North American freeways (known as parkways) opened in the New York City area in the 1920s. Britain, heavily influenced by the railways, did not build its first motorway, the Preston By-pass (M6), until 1958.

Definition standards

An aerial view of Finnish national road 3 (E12), a motorway between Tampere and Helsinki in Finland

The west coast motorway E6/E20 in central Gothenburg, Sweden

There are several international standards that give some definitions of words such as motorways, but there is no formal definition of the English language words such as freeway, motorway, and expressway, or of the equivalent words in other languages such as autoroute, Autobahn, autostrada, autocesta, that are accepted worldwide—in most cases these words are defined by local statute or design standards or regional international treaties. Descriptions that are widely used include:

Vienna Convention on Road Signs and Signals

"Motorway" means a road specially designed and built for motor traffic that does not serve properties bordering on it, and that:

1. Is provided, except at special points or temporarily, with separate carriageways for the two directions of traffic, separated from each other either by a dividing strip not intended for traffic or, exceptionally, by other means;
2. Does not cross at level with any road, railway or tramway track, or footpath; and,
3. Is specially sign-posted as a motorway;

One green or blue symbol (like ) appears at motorway entry in countries that follow the Vienna Convention. Exits are marked with another symbol: .

The definitions of "motorway" from the OECD and PIARC are almost identical.

British Standards

Motorway: Limited-access dual carriageway road, not crossed on the same level by other traffic lanes, for the exclusive use of certain classes of motor vehicle.

ITE (including CITE)

Freeway: A divided major roadway with full control of access and with no crossings at grade. This definition applies to toll as well as toll-free roads.

Freeway A: This designates roadways with greater visual complexity and high traffic volumes. Usually this type of freeway will be found in metropolitan areas in or near the central core and will operate through much of the early evening hours of darkness at or near design capacity.

Freeway B: This designates all other divided roadways with full control of access where lighting is needed.

In the European Union, for statistical and safety purposes, some distinction might be made between motorway and expressway. For instance a principal arterial might be considered as:

Roads serving long distance and mainly interurban movements. Includes motorways (urban or rural) and expressways (road which does not serve properties bordering on it and which is provided with separate carriageways for the two directions of traffic). Principal arterials may cross through urban areas, serving suburban movements. The traffic is characterized by high speeds and full or partial access control (interchanges or junctions controlled by traffic lights). Other roads leading to a principal arterial are connected to it through side collector roads.

In this view, CARE's definition stands that a motorway is understood as a

public road with dual carriageways and at least two lanes each way. All entrances and exits are signposted and all interchanges are grade separated. Central barrier or median present throughout the road. No crossing is permitted, while stopping is permitted only in an emergency. Restricted access to motor vehicles, prohibited to pedestrians, animals, pedal cycles, mopeds, agricultural vehicles. The minimum speed is not lower than 50 km/h [31 mph] and the maximum speed is not higher than 130 km/h [81 mph] (except Germany where no speed limit is defined).

Motorways are designed to carry heavy traffic at high speed with the lowest possible number of accidents. They are also designed to collect long-distance traffic from other roads, so that conflicts between long-distance traffic and local traffic are avoided. According to the common European definition, a motorway is defined as "a road, specially designed and built for motor traffic, which does not serve properties bordering on it, and which: (a) is provided, except at special points or temporarily, with separate carriageways for the two directions of traffic, separated from each other, either by a dividing strip not intended for traffic, or exceptionally by other means; (b) does not cross at level with any road, railway or tramway track, or footpath; (c) is specially sign-posted as a motorway and is reserved for specific categories of road motor vehicles." Urban motorways are also included in this definition. However, the respective national definitions and the type of roads covered may present slight differences in different EU countries.

History

Historical map of the original A8-A9 motorway, Italy. The first motorway ever built in the world was opened on 21 September 1924.

The first version of modern controlled-access highways evolved during the first half of the 20th century. The Long Island Motor Parkway on Long Island, New York, opened in 1908 as a private venture, was the world's first limited-access roadway. It included many modern features, including banked turns, guard rails and reinforced concrete tarmac.

Modern controlled-access highways originated in the early 1920s in response to the rapidly increasing use of the automobile, the demand for faster movement between cities and as a consequence of improvements in paving processes, techniques and materials. These original high-speed roads were referred to as "dual highways" and have been modernized and are still in use today.

The first dual highway opened in Italy in 1924, between Milan and Varese, and now forms parts of the A8 and A9 motorways. This highway, while divided, contained only one lane in each direction and no interchanges. Shortly thereafter, in New York in 1924, the Bronx River Parkway was opened to traffic. The Bronx River Parkway was the first road in North America to utilize a median strip to separate the opposing lanes, to be constructed through a park and where intersecting streets crossed over bridges. The Southern State Parkway opened in 1927, while the Long Island Motor Parkway was closed in 1937 and replaced by the Northern State Parkway (opened 1931) and the contiguous Grand Central Parkway (opened 1936). In Germany, construction of the Bonn-Cologne Autobahn began in 1929 and was opened in 1932 by Konrad Adenauer, then the mayor of Cologne.

In Canada, the first precursor with semi-controlled access was The Middle Road between Hamilton and Toronto, which featured a median divider between opposing traffic flow, as well as the nation's first cloverleaf interchange. This highway developed into the Queen Elizabeth Way, which featured a cloverleaf and trumpet interchange when it opened in 1937, and until the Second World War, boasted the longest illuminated stretch of roadway built. A decade later, the first section of Highway 401 was opened, based on earlier designs. It has since gone on to become the busiest highway in the world.

The word freeway was first used in February 1930 by Edward M. Bassett. Bassett argued that roads should be classified into three basic types: highways, parkways, and freeways. In Bassett's zoning and property law-based system, abutting property owners have the rights of light, air and access to highways, but not parkways and freeways; the latter two are distinguished in that the purpose of a parkway is recreation, while the purpose of a freeway is movement. Thus, as originally conceived, a freeway is simply a strip of public land devoted to movement to which abutting property owners do not have rights of light, air or access.

Design

Highway interchange between the Dolphin Expressway and Palmetto Expressway in Greater Miami, Florida, United States

Highway 401 in Southern Ontario, Canada. An example of a collector-express freeway design, the route features four carriageways through Toronto.

The A10 near Orléans, France, showing hard shoulder and emergency telephone. The broken demarcation line for the hard shoulder is specific to France, and serves as a safety reference mark for drivers: the advisory distance from the vehicle ahead is two dashes minimum.

Freeways, by definition, have no at-grade intersections with other roads, railroads or multi-use trails. Therefore, no traffic signals are needed and through traffic on freeways does not normally need to stop at traffic signals. Some countries, such as the United States, allow for limited exceptions: some movable bridges, for instance the Interstate Bridge on Interstate 5 between Oregon and Washington, do require drivers to stop for ship traffic.

The crossing of freeways by other routes is typically achieved with grade separation either in the form of underpasses or overpasses. In addition to sidewalks (pavements) attached to roads that cross a freeway, specialized pedestrian footbridges or tunnels may also be provided. These structures enable pedestrians and cyclists to cross the freeway at that point without a detour to the nearest road crossing.

Access to freeways is typically provided only at grade-separated interchanges, though lower-standard right-in/right-out (left-in/left-out in countries that drive on the left) access can be used for direct connections to side roads. In many cases, sophisticated interchanges allow for smooth, uninterrupted transitions between intersecting freeways and busy arterial roads. However, sometimes it is necessary to exit onto a surface road to transfer from one freeway to another. One example in the United States (notorious for the resulting congestion) is the connection from Interstate 70 to the Pennsylvania Turnpike (Interstate 70 and Interstate 76) through the town of Breezewood, Pennsylvania.

Speed limits are generally higher on freeways and are occasionally nonexistent (as on much of Germany's Autobahn network). Because higher speeds reduce decision time, freeways are usually equipped with a larger number of guide signs than other roads, and the signs themselves are physically larger. Guide signs are often mounted on overpasses or overhead gantries so that drivers can see where each lane goes. Exit numbers are commonly derived from the exit's distance in miles or kilometers from the start of the freeway. In some areas, there are public rest areas or service areas on freeways, as well as emergency phones on the shoulder at regular intervals.

In the United States, mileposts usually start at the southern or westernmost point on the freeway (either its terminus or the state line). California, Ohio and Nevada use postmile systems in which the markers indicate mileage through the state's individual counties. However, Nevada and Ohio also use the standard milepost system concurrently with their respective postmile systems. California numbers its exits off its freeways according to a milepost system but does not use milepost markers.

Diagram showing lanes and road layout (Irish road markings)

In Europe and some other countries, motorways typically have similar characteristics such as:

• A typical design speed in the range of 100–130 km/h (62–81 mph)
• Minimum values for horizontal curve radii around 750 to 900 m (2,460 to 2,950 ft).
• Maximum longitudinal gradients typically not exceeding 4% to 5%.
• Cross sections incorporating a minimum of two through-traffic lanes for each direction of travel, with a typical width of 3.50 to 3.75 m (11 ft 6 in to 12 ft 4 in) each, separated by a central median.
• An obstacle-free zone varying from 4.5 to 10 m (15 to 33 ft), or alternatively installation of appropriate vehicle restraint systems.
• Proper design of grade-separated interchanges to provide for the movement of traffic between two or more roadways on different levels.
• More frequent (compared to other road types) construction of tunnels and overpasses, requiring complex equipment and methods of operation.
• Installation of highly efficient road equipment and traffic control devices.

Cross sections

Two-lane freeways, often undivided, are sometimes built when traffic volumes are low or right-of-way is limited; they may be designed for easy conversion to one side of a four-lane freeway. (For example, most of the Bert T. Combs Mountain Parkway in eastern Kentucky is two lanes, but work has begun to make all of it four-lane.) These are often called Super two roads. Several such roads are infamous for a high rate of lethal crashes; an outcome because they were designed for short sight distances (sufficient for freeways without oncoming traffic, but insufficient for the years in service as two-lane road with oncoming traffic). An example of such a "Highway to Hell" was E4 from Gävle to Axmartavlan, Sweden. The high rate of crashes with severe personal injuries on that (and similar) roads did not cease until a median crash barrier was installed, transforming the fatal crashes into non-fatal crashes. Otherwise, freeways typically have at least two lanes in each direction; some busy ones can have as many as 16 or more lanes in total.

In San Diego, California, Interstate 5 has a similar system of express and local lanes for a maximum width of 21 lanes on a 2-mile (3.2 km) segment between Interstate 805 and California State Route 56. In Mississauga, Ontario, Highway 401 uses collector-express lanes for a total of 18 lanes through its intersection with Highway 403/Highway 410 and Highway 427.

These wide freeways may use separate collector and express lanes to separate through traffic from local traffic, or special high-occupancy vehicle lanes, either as a special restriction on the innermost lane or a separate roadway, to encourage carpooling. These HOV lanes, or roadways open to all traffic, can be reversible lanes, providing more capacity in the direction of heavy traffic, and reversing direction before traffic switches. Sometimes a collector/distributor road, a shorter version of a local lane, shifts weaving between closely spaced interchanges to a separate roadway or altogether eliminates it.

In some parts of the world, notably parts of the US, frontage roads form an integral part of the freeway system. These parallel surface roads provide a transition between high-speed "through" traffic and local traffic. Frequent slip-ramps provide access between the freeway and the frontage road, which in turn provides direct access to local roads and businesses.

Except on some two-lane freeways (and very rarely on wider freeways), a median separates the opposite directions of traffic. This strip may be as simple as a grassy area, or may include a crash barrier such as a "Jersey barrier" or an "Ontario Tall Wall" to prevent head-on collisions. On some freeways, the two carriageways are built on different alignments; this may be done to make use of available corridors in a mountainous area or to provide narrower corridors through dense urban areas.

Control of access

Control of access relates to a legal status which limits the types of vehicles that can use a highway, as well as a road design that limits the points at which they can access it.

Major arterial roads will often have partial access control, meaning that side roads will intersect the main road at grade, instead of using interchanges, but driveways may not connect directly to the main road, and drivers must use intersecting roads to access adjacent land. At arterial junctions with relatively quiet side roads, traffic is controlled mainly by two-way stop signs which do not impose significant interruptions on traffic using the main highway. Roundabouts are often used at busier intersections in Europe because they help minimize interruptions in flow, while traffic signals that create greater interference with traffic are still preferred in North America. There may be occasional interchanges with other major arterial roads. Examples include US 23 between SR 15's eastern terminus and Delaware, Ohio, along with SR 15 between its eastern terminus and I-75, US 30, SR 29/US 33, and US 35 in western and central Ohio. This type of road is sometimes called an expressway.

Non-motorized access on freeways

Main article: Non-motorized access on freeways

Freeways are usually limited to motor vehicles of a minimum power or weight; signs may prohibit cyclists, pedestrians and equestrians and impose a minimum speed. It is possible for non-motorized traffic to use facilities within the same right-of-way, such as sidewalks constructed along freeway-standard bridges and multi-use paths next to freeways such as the Suncoast Trail along the Suncoast Parkway in Florida.

De Lucht Rest Area on the Dutch A2 - a typical rest area in the Netherlands with services (fuel, refreshments and toilets). The only access is via the highway that it serves.

In some US jurisdictions, especially where freeways replace existing roads, non-motorized access on freeways is permitted. Different states of the United States have different laws. Cycling on freeways in Arizona may be prohibited only where there is an alternative route judged equal or better for cycling. Wyoming, the second least densely populated state, allows cycling on all freeways. Oregon allows bicycles except on specific urban freeways in Portland and Medford.

In countries such as the United Kingdom new motorways require an Act of Parliament to ensure restricted right of way. Since upgrading an existing road (the "King's Highway") to a full motorway will result in extinguishing the right of access of certain groups such as pedestrians, cyclists and slow-moving traffic, many controlled access roads are not full motorways. In some cases motorways are linked by short stretches of road where alternative rights of way are not practicable such as the Dartford Crossing (the furthest downstream public crossing of the River Thames) or where it was not economic to build a motorway alongside the existing road such as the former Cumberland Gap. The A1 is a good example of piece-wise upgrading to motorway standard—as of January 2013, the 639-kilometer-long (397 mi) route had five stretches of motorway (designated as A1(M)), reducing to four stretches in March 2018 with completion of the A1(M) through North Yorkshire.

Construction techniques

The Belgrade Urban Motorway, constructed between 1970 and 1977, required demolitions of streets and houses, characteristic of urban motorways. In Novi Beograd, the motorway path was already laid out, requiring no demolitions.

The most frequent way freeways are laid out is by building them from the ground up after obstructions such as forestry or buildings are cleared away. Sometimes they deplete farmland, but other methods have been developed for economic, social and even environmental reasons.

Full freeways are sometimes made by converting at-grade expressways or by replacing at-grade intersections with overpasses; however, in the US, any at-grade intersection that ends a freeway often remains an at-grade intersection. Often, when there is a two-lane undivided freeway or expressway, it is converted by constructing a parallel twin corridor, and leaving a median between the two travel directions. The median-side travel lane of the old two-way corridor becomes a passing lane.

Other techniques involve building a new carriageway on the side of a divided highway that has a lot of private access on one side and sometimes has long driveways on the other side since an easement for widening comes into place, especially in rural areas.

When a third carriageway is added, sometimes it can shift a directional carriageway by 20–60 meters (50–200 ft) (or maybe more depending on land availability) as a way to retain private access on one side that favors over the other. Other methods involve constructing a service drive that shortens the long driveways (typically by less than 100 meters (330 ft)).

Interchanges and access points

The High Five Interchange in Dallas, Texas, a stack interchange with elevated entrance and exit ramps connecting Interstate 635 and U.S. Route 75

An aerial view of the Lakalaiva interchange in the Tampere Ring Road between the Highway 3 (E12) and Highway 9 (E63) near city of Tampere

Main article: Interchange (road)

An interchange or a junction is a highway layout that permits traffic from one controlled-access highway to access another and vice versa, whereas an access point is a highway layout where traffic from a distributor or local road can join a controlled-access highway. Some countries, such as the United Kingdom, do not distinguish between the two, but others make a distinction; for example, Germany uses the words Kreuz ("cross") or Dreieck ("triangle") for the former and Ausfahrt ("exit") for the latter. In all cases one road crosses the other via a bridge or a tunnel, as opposed to an at-grade crossing.

The inter-connecting roads, or slip-roads, which link the two roads, can follow any one of a number of patterns. The actual pattern is determined by a number of factors including local topology, traffic density, land cost, building costs, type of road, etc. In some jurisdictions feeder/distributor lanes are common, especially for cloverleaf interchanges; in others, such as the United Kingdom, where the roundabout interchange is common, feeder/distributor lanes are seldom seen.

Motorways in Europe typically differ between exits and junctions. An exit leads out of the motorway system, whilst a junction is a crossing between motorways or a split/merge of two motorways. The motorway rules end at exits, but not at junctions. However, on some bridges, motorways, without changing appearance, temporarily end between the two exits closest to the bridge (or tunnel), and continues as dual carriageways. This is in order to give slower vehicles a possibility to use the bridge. The Queen Elizabeth II Bridge / Dartford tunnel at London Orbital is an example of this. London Orbital or the M25 is a motorway surrounding London, but at the last River Thames crossing before its mouth, motorway rules do not apply. (At this crossing the London Orbital is labeled A282 instead.)

A few of the more common types of junction are shown below:

Various interchange layouts

• 

  Four-level stack: used as a major junction, usually for freeway junctions

• 

  Roundabout interchange: very common in the United Kingdom as either a junction or exit

• 

  Cloverleaf: used mainly as a junction

• 

  Parclo (partial cloverleaf) interchange: often used to link a minor road with a junction

• 

  Trumpet interchange: a motorway "T" junction, used where the interchange represents the terminus of one of the two roads; also common on toll roads as it requires only one tollbooth

• 

  Motorway split or merge; basic logic resembles the T-junction

• 

  Single-point urban interchange, or SPUI; used in dense urban areas to reduce the land footprint of the junction

• 

  Diverging diamond interchange; designed to improve traffic flow and safety by minimizing turns that must cross oncoming lanes of traffic

Safety

There are many differences between countries in their geography, economy, traffic growth, highway system size, degree of urbanization and motorization, etc.; all of which need to be taken into consideration when comparisons are made. According to some EU papers, safety progress on motorways is the result of several changes, including infrastructure safety and road user behavior (speed or seat belt use), while other matters such as vehicle safety and mobility patterns have an impact that has not been quantified.

Motorways compared with other roads

Motorways are the safest roads by design. While accounting for more than one quarter of all kilometres driven, they contributed only 8% of the total number of European road deaths in 2006. Germany's Federal Highway Research Institute provided International Road Traffic and Accident Database (IRTAD) statistics for the year 2010, comparing overall fatality rates with motorway rates (regardless of traffic intensity):

Killed per 1 billion veh·km

Country	All roads	Motorways
Austria	7.32	2.15
Belgium	8.51	2.87
Czech Republic	16.22	3.38
Denmark	5.65	1.92
Finland	5.05	0.61
France	7.12	1.79
Germany	5.18	1.98
Slovenia	7.74	3.77
Switzerland	5.25	1.04
United States	6.87	3.62
Sources:

The German autobahn network illustrates the safety trade-offs of controlled access highways. The injury crash rate is very low on autobahns, while 22 people died per 1,000 injury crashes—although autobahns have a lower rate than the 29 deaths per 1,000 injury accidents on conventional rural roads, the rate is higher than the risk on urban roads. Speeds are higher on rural roads and autobahns than urban roads, increasing the severity potential of a crash.

According to ETSC, German motorways without a speed limit, but with a 130 km/h (81 mph) speed recommendation, are 25% more deadly than motorways with a speed limit.

Germany also introduced some 130 km/h (81 mph) speed limits on various motorway sections which were not limited. This generated a reduction in deaths in a range from 20% to 50% on those sections.

Road class	Injury crashes	Fatalities	Injuries	Fatalities	Fatalities
			per 1,000,000,000 km travel		per 1000 injury crashes
Autobahn	17,847	387	80	1.7	21.7
Urban	206,696	1,062	1390	5.1	3.7
Rural	75,094	2,151	220	7.6	34.5
Total	299,637	3,600	420	5.0	12.0

Causes of accidents

Speed, in Europe, is considered to be one of the main contributory factors to collisions. Some countries, such as France and Switzerland, have achieved a death reduction by a better monitoring of speed. Tools used for monitoring speed might be an increase in traffic density; improved speed enforcement and stricter regulation leading to driver license withdrawal; safety cameras; penalty point; and higher fines. Some other countries use automatic time-over-distance cameras (also known as section controls) to manage speed.

Fatigue is considered as a risk factor more specific to monotonous roads such as motorways, although such data are not monitored/recorded in many countries. According to Vinci Autoroutes, one third of accidents in French motorways are due to sleepy driving.

23% of people killed on French motorways were not wearing seat belts, while 98% of front-seat passengers and 87% of rear-seat passenger wear seat belts.

Fatalities trends

Although safety results do not change much from year to year, in Europe some changes have been observed: motorway fatalities decreased by 41% during the 2006–2015 decade, but increased by 10% between 2014 and 2015. However, taking into account motorway network length to reflect exposure, data shows that fatalities per thousand kilometres halved between 2006 and 2015.

EU motorway fatalities

Year	Fatalities	Rate per million population	Rate per 1,000 km of motorways
2006	3,485	7.1	54.4
2010	2,329	4.7	32.9
2015	2,048	4.1	27.3
Source: Traffic Safety Basic Facts 2017, Motorways

Toll effect

Main article: Shunpiking

A University of Barcelona study suggests that if tolls are implemented on a controlled-access highway, drivers may seek alternative routes to avoid paying the tolls. This may result in a decrease of safety on roads which are not designed for heavy traffic.

Safety in urban areas

In the United Kingdom, there are very few studies regarding the impact of road traffic accidents from existing and new urban motorways. In particular, new urban motorways do not grant a reduction of traffic accidents.

In Italy, a study performed on urban motorway A56 Tangenziale di Napoli showed that reduction of speed leads to a decrease in accidents.

In Marseille, France, from June 2009 to May 2010, CEREMA, the French centre for studies on risk, mobility and environment, performed a study on Marius, a network of urban motorways. This study established a link between accidents and traffic variables:

• for single vehicle accidents, the 6-minute average speed on the fast lane; and the time headway (on every lane),
• for multiple vehicle accidents, the occupancy, and the time headway (for the middle lane).

The 150-kilometer-long (93 mi) Marius network counts 292 injury accidents or fatalities for 1.5 billion of vehicle-kilometres, that is 189 injury accidents or fatalities for 1 billion of vehicle-kilometres.

Some European countries have improved safety of urban motorways, with a set of measures to dynamically manage traffic flow in response to changing volumes, speeds, and incidents, including:

• Variable speed limits, line control, and speed harmonization
• Shoulder running with emergency refuge areas
• Queue warning and variable messaging
• 24/7 monitoring of traffic with cameras and/or in-pavement sensors (both to detect incidents and identify when to reduce speed limits)
• Incident management
• Automated enforcement
• Specialized algorithms for temporary shoulder running, variable speed limits, and/or incident detection and management
• Ramp metering (coordinated or independent function)

In 1994, it was assumed that lighting urban motorways would benefit from more safety than unlighted ones.

In California, in 2001, a study established, for urban freeways, some relationships among urban freeway accidents, traffic flow, weather, and lighting conditions:

• it establishes a difference between dry freeways in daylight and wet freeways in darkness
• it establishes that left-lane collisions are more likely induced by volume effects, while right-lane collisions are more closely tied to speed variances in adjacent lanes (in California, people drive in the right lane except when passing).

Environmental effects

Traffic congestion, such as this on the Downtown Connector in Atlanta, is a cause of photochemical smog.

Highway lighting can have a negative influence on those living close to the freeway. High-mast lighting is an alternative as it concentrates the light on the road, but the tall structures can also lead to a NIMBY effect. Seen here is Ontario Highway 401 through suburban Toronto.

Tunnel on the A1 motorway in Greece

Controlled-access highways have been constructed both between major cities as well as within them, leading to the sprawling suburban development found near most modern cities. Highways have been heavily criticized by environmentalists, urbanists, and preservationists for the noise, pollution, and economic shifts they bring. Additionally, they have been criticized by the driving public for the inefficiency with which they handle peak hour traffic.

Often, rural highways open up vast areas to economic development and municipal services, generally raising property values. In contrast to this, above-grade highways in urban areas are often a source of lowered property values, contributing to urban decay. Even with overpasses and underpasses, neighbourhoods are divided—especially impoverished ones where residents are less likely to own a car, or to have the political and economic influence to resist construction efforts. Beginning in the early 1970s, the US Congress identified freeways and other urban highways as responsible for most of the noise exposure of the US population. Subsequently, computer models were developed to analyze freeway noise and aid in their design to help minimize noise exposure.

Some cities have implemented freeway removal policies, under which freeways have been demolished and reclaimed as boulevards or parks, notably in Seoul (Cheonggyecheon), Portland (Harbor Drive), New York City (West Side Elevated Highway), Boston (Central Artery), San Francisco (Embarcadero Freeway), Seattle (Alaskan Way Viaduct), and Milwaukee (Park East Freeway).

An alternative to surface or above-ground freeway construction has been the construction of underground urban freeways using tunnelling technologies. This has been employed in Madrid and Prague, as well as the Australian cities of Sydney (which has five such freeways), Brisbane (which has three), and Melbourne (which has two). This has had the benefit of not creating heavily trafficked surface roads and, in the case of Melbourne's EastLink freeway, prevented the destruction of an ecologically sensitive area.

Other Australian cities face similar problems (lack of available land, cost of home acquisition, aesthetic problems and community opposition). Brisbane, which also has to contend with physical boundaries (the Brisbane River) and rapid population increases, has embraced underground freeways. There are currently three open to traffic (Clem Jones Tunnel (CLEM7), Airport Link and Legacy Way) and one (East-West Link) is currently in planning. All of the tunnels are designed to act as an inner-city ring road or bypass system and include provision for public transport, whether underground or in reclaimed space on the surface. However, freeways are not beneficial for road-based public transport services, because the restricted access to the roadway means that it is awkward for passengers to get to the limited number of boarding points unless they drive to them, largely defeating the purpose.

In Canada, the extension of Highway 401 toward Detroit, known as the Herb Gray Parkway, has been designed with numerous tunnels and underpasses that provide land for parks and recreational uses.

Freeway opponents have found that freeway expansion is often self-defeating: expansion simply generates more traffic. That is, even if traffic congestion is initially shifted from local streets to a new or widened freeway, people will begin to use their cars more and commute from more remote locations. Over time, the freeway and its environs become congested again as both the average number and distance of trips increases. This phenomenon is known as induced demand.

Urban planning experts such as Drusilla Van Hengel, Joseph DiMento and Sherry Ryan argue that although properly designed and maintained freeways may be convenient and safe, at least in comparison to uncontrolled roads, they may not expand recreation, employment and education opportunities equally for different ethnic groups, or for people located in certain neighborhoods of any given city. Still, they may open new markets to some small businesses.

Construction of urban freeways for the US Interstate Highway System, which began in the late 1950s, led to the demolition of thousands of city blocks and the dislocation of many more thousands of people. The citizens of many inner city areas responded with the freeway and expressway revolts. Through the study of Washington's response, it can be shown that the most effective changes came not from executive or legislative action, but instead from policy implementation. One of the foremost rationales for the creation of the United States Department of Transportation (USDOT) was that an agency was needed to mediate between the conflicting interests of interstates and cities. Initially, these policies came as regulation of the state highway departments. Over time, USDOT officials re-focused highway building from a national level to the local scale. With this shift of perspective came an encouragement for alternative transportation, and locally based planning agencies.

At present, freeway expansion has largely stalled in the United States, due to a multitude of factors that converged in the 1970s: higher due process requirements prior to taking of private property, increasing land values, increasing costs for construction materials, local opposition to new freeways in urban cores, the passage of the National Environmental Policy Act (which imposed the requirement that each new federally funded project must have an environmental impact statement or report) and falling gas tax revenues as a result of the nature of the flat-cent tax (it is not automatically adjusted for inflation), the tax revolt movement, and growing popular support for high-speed mass transit in lieu of new freeways.

See also

• List of controlled-access highway systems

External links

• Media related to Controlled-access highway at Wikimedia Commons