A New Perspective on Road Design

by: Michael J. Wallwork, P.E.

    Roads have many functions. Some transport people along with thousands of tons of goods great distances at high speeds between states other simply allow people to walk around the corner to visit a neighbor.

    To fulfills these and many other functions, our highways and streets need to be designed not with one generic set of standards but with standards that are appropriate for the function of each road. In most areas of this country, roads are designed with a basic philosophy to provide for high volume, high speed vehicle flow, often without any facilities for bicyclists and pedestrians. As some engineers say money spent on enhancements such as sidewalks, bicycle lanes, landscaping etc. is money wasted. Very few engineers try to limit vehicle speeds so that pedestrians and bicyclists can also use roads safely. In some areas local streets are designed to freeway standards (50 feet wide). Other cities permit streets to be 10 feet wide.

    Geometric Design of Streets and Highways, the manual of the American Association of State Highway Officials (AASHTO) and the "Manual of Uniform Traffic Control Devices" (MUTCD) are the two main standards that engineers use when designing roads. These documents are very much vehicle-orientated manuals with limited consideration given to the needs of other roadway users. Engineers often use these standards to prevent the use of alternate designs for streets and highways. The AASHTO Manual does point out the need for channelizing, which creates compact intersections, and the need for sidewalks to be at or as near as possible to the right-of-way line.

    Before looking at road design standards, we should first review the five basic types and functions of roads.

Freeways	High-speed, high volume, long-distance, often interstate travel by cars and trucks.
Arterial	High-speed, high volume, long-distance, inter-city, cross-town travel by cars and trucks with some long-distance bicycle travel.
Secondary Arterial Roads	Relatively short distributive travel by cars and trucks with greater bicyclist use.
Collector Roads	Collect and distribute vehicles to and from the arterial road system to residential neighborhoods with relatively high pedestrian and bicycle use.
Residential Streets	Access to homes with high use by vehicles, pedestrians and bicyclists.

Design Philosophies

When designing a road network there are two options available:

1. Cul-de-sac/Single Entrance subdivisions accessing a single arterial road.

This style of design is used to:

	allow some people to live on cul-de-sacs with little traffic, while requiring those who live on the collector roads servicing the cul-de-sacs to live with high volumes of traffic;
	restrict access into the sub-divisions for outsiders. In doing so, they severely restrict access to arterial roads for residents and overload arterial roads with traffic, especially turning vehicles.

2.  A grid network of streets distributes traffic throughout the neighborhood so no residents suffer from high traffic levels and all residents enjoy high levels of mobility. Because residents can reach locations by foot or bicycle, vehicle volumes are reduced. The many entrances and exits reduce traffic on arterial roads, especially turning vehicles, thereby reducing road costs. To be truly effective in reducing vehicle speeds, grid neighborhoods must be built with traffic calming elements integrated into the street design. An existing neighborhood can be retrofitted with these elements.

Within each road type and function there are two different design philosophies:

Freeways

	High-speed, high volume roads devoid of most vegetation and gray concrete.
	High-speed, high volume roads with landscaping and tinted concrete.

Arterial Roads

	Wide, open, high-speed, treeless design with two-way, left-turn lanes, raised or grassed medians and usually no sidewalks or bicycle lanes.
	Narrow, tree-lined boulevards with trees in the medians, nature strips with sidewalks located behind them, bicycle lanes and street lighting for pedestrians and drivers. (Nature strips are the six to seven feet of green space between sidewalks and the curb and gutter which separate the people on the sidewalk from the traffic by providing space for trees and nature.)

Secondary Arterial Roads

Wide, open, high-speed, treeless design, raised or grassed medians and often no sidewalks or bicycle lanes.

Narrow, tree-lined boulevards with trees in the medians, nature strips with sidewalks located behind them, bicycle lanes, street lighting for pedestrians and drivers and bulb-outs with on-street parking. (Bulb-outs are an extension of the curb and gutter to the edge of the parking lane. The space created can be used for landscaping and street furniture and to reduce pedestrian crossing distances and slow vehicles.)

Collector Roads

	Wide, open, treeless roads , no sidewalks or bicycle lanes.
	Narrow, tree-lined, on street parking with pedestrian refuges, sidewalks, bicycle lanes and built-in traffic calming elements.

Residential Streets

	Wide (up to 50 feet wide), sweeping curves, no on street parking, treeless, no sidewalks, rollover curbs so cars can park on the sidewalks.
	Usually 24 feet wide, but can be only 10 feet wide if only servicing a few homes, parking permitted on both sides, short (no more than a quarter mile long), sidewalks and trees on both sides, and built-in traffic calming elements.

Intersection Control

As with road design there are two different philosophies used to design intersections. Intersections are, by nature, areas of high-speed conflicts. These philosophies provide for:

wide, open, high-speed intersections using very large right-turn and left-turn radii.

- or -

compact, low speed intersections using minimum size radii, or modern roundabouts.

    The operational difference between these two is that the compact intersections are more efficient. The reason is very simple. When moving through, or turning at a compact intersection, the driver is within the intersection for a smaller time period. Therefore, the clearance times and pedestrian crosswalk times can be reduced. The resulting shorter cycle time reduces delays to all road users, especially pedestrians. For example, to cross a residential street that is 24 feet wide which has a radii of 15 feet and a sidewalk located six feet back from the curb a gutter, pedestrians have to cross 26 feet of road. If you move the sidewalk to the back of the curb and increase the corner radii to 50 feet, pedestrians have to cross 89 feet of road. In this situation, pedestrians also have to contend with vehicles turning at a higher rate of speed due to the larger radius.

    The other component of intersection design addresses the type of traffic control to use. Traffic control devices are designed to either separate drivers in a timely fashion (traffic signals) or to assign priority (traffic signs). Current traffic control devices are:

1. Traffic signals separate vehicles in time only.

	Drivers running the red light often causing major accidents and fatalities. Engineers have known for many years that traffic signals only change the types of accidents, not necessarily the number of accidents. Although they can reduce the number of right-angle accidents, the number of left-turn and rear-end accidents is increased.
	They are expensive to operate and maintain, each costing approximately $3,000 per year for electricity, globe replacement, breakdowns, loop failures, etc.
	They can create a liability for the operator for often the traffic signal, not the driver, is blamed for accidents at signalized intersections. Modern traffic signals have conflict monitors to prevent two opposing green signals from being displayed.
	They can have a significant role in managing traffic. That is, as a part of a coordinated traffic signal system, they can move significant volumes of traffic along major roads. However, in doing so they significantly delay drivers on side streets feeding into the major road.

2. Roundabouts are proven to be the safest form of traffic control.

	Many years of research from around the world has proven roundabouts to have 50 to 90 percent fewer traffic accidents than all other forms of traffic control.
	They are virtually maintenance free, except for the landscaping.
	They have a higher vehicle capacity than stop signs or traffic signals.
	They will slow traffic to less than 12 mph if designed well.
	They may act as a gateway to communities and cities.
	They have much less delay and pollution than other forms of traffic control.
	The modern roundabout is smaller, safer, slower, and has a much higher vehicle capacity than rotaries or traffic circles.

3. Stop signs assign priority.

	Because they are simply signs, stop signs are frequently violated. According to the Federal Highway Administration, 68% of drivers do not stop at stop signs. Yet many people favor stop signs because they are inexpensive.
	They can be a liability for the operator because some of the features of the sign are blamed for causing accidents instead of driver error.

4. Yield signs assign priority.

An under-utilized device which, when the sight distance requirements are met, is more efficient than a stop sign. Yield signs suffer from a poor reputation in this country because it had been argued that if the yield sign had been a stop sign then the crash would not have occurred. In other countries the yield sign is the control of choice as it does not require the driver to stop. The stop sign is the control of last resort to be used when there is inadequate sight distance.

Transportation Customers

    People who require transportation include vehicle drivers as well as children walking to a friends' houses, elderly or disabled people who cannot drive and people who prefer walking and/or bicycling to driving. Many road users are overlooked when roads are designed. Roads should be built to service all members in a community, not just drivers. How well user demand is addressed provides a clear idea of where a community's priority lies. Only about 68% of people drive vehicles, the others are either too young, too old, have a disability, or are too poor to own a vehicle. Communities that do not provide sidewalks, bicycle lanes and curb ramps are discriminating against the non-drivers. Roads designed with minimum road widths, sidewalks, bicycle lanes, landscaping, and to the standards of the American with Disabilities Act (ADA), can provide mobility for all people. Why should drivers be provided with unlimited mobility on roads while non-drivers are given nothing except a harsh, unsafe environment? Aren't't we all equal?

There is a change coming

    Some cities in the USA, including North Hampton, MA and Belmont, NC, have outlawed the cul-de-sac because they limit mobility. Many European cities have closed their downtown's to the automobile and have prospered by the increased mobility of pedestrians. In England the Ministry of Transportation recently put a hold on all new road construction. They found that new roads, by virtue of their construction, generated a 20 percent increase in vehicle trips. Other cities have revitalized themselves by turning away from the traditional high speed, high volume designs to narrow, tree-lined, pedestrian/bicycle friendly main streets, arterial and local streets.

The choice is yours!

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	the wide, open, featureless road or the tree lined, pedestrian/bicycle friendly boulevards.
	the wide, open, high-speed intersections or smaller, compact intersections
	modern attractive roundabouts for traffic control or the scourge of many drivers, the traffic signal.

WHICH WILL YOU CHOOSE?