To design a road to fit the needs, we need to know how it is used. These traffic characteristics include the
types of traffic on the road, how many use the road on an average day, and how fast they travel.
Roads are classified based on the role they play in the transportation network:
A road’s functional class is a factor in design decisions. More design effort and money is spent on
higher functional classes. For example, lanes on freeways are wider than lanes on local roads. If you
are not sure what functional class a road belongs in, ask your NYSDOT Planning office or metropolitan
planning organization (MPO) for assistance.
The way land is used near the road will affect traffic on the road, and the design of the road itself.
Common examples are rural roads with light, high-speed traffic and ditches, or urban streets with
congested, low-speed traffic, closed drainage, and curbs.
Land use affects the amount and type of traffic the road carries. Roads in agricultural areas should be
wide enough for the farm machinery that uses the road. Commercial areas will get more trucks and
may need wider streets. Residential neighborhood roads can be designed for slower speeds than rural
One pitfall to avoid is classifying a road solely on land use. The amount and type of traffic has to be
taken into account. A residential street that services a larger area may need to be classified as a collector
or even a minor arterial.
Stopping sight distance
Stopping sight distance is the distance traveled from the instant a driver sees a problem until the vehicle
stops. It is the distance taken up by the information, decision, and action phases of driving. In addition
to reaction time, it takes time to stop once the brakes are applied. The driver needs to see objects in the
road far enough away to come to a controlled stop before hitting them. Table 1 shows stopping sight
distance on wet pavement. Because of gravity, braking distances are longer on downhill sections and shorter on uphill sections. Sometimes, meeting the specified distances is not enough.
The reaction time used to calculate stopping sight distance is two and one half seconds. Where
complex decisions or maneuvers are required, longer reaction times are needed. On slippery
pavements or unpaved roads, braking distances are likely to be longer.
Stopping sight distance for grades other than those shown in Table 1 can be calculated
using this equation:
Trucks need more distance to stop than other vehicles, but truck drivers can usually see farther
because the seat is higher. On hillcrests, this is usually enough to compensate, but on horizontal
curves, sight obstructions are often high enough that truck drivers cannot see over them. When
horizontal curves occur on steep downhill sections, the higher driver’s eye height often is not
enough to compensate for the longer stopping distances. In these cases, more than the minimum
stopping sight distance should be provided.
On low-volume roads, less sight distance may be needed since the chances of a multi-vehicle
accident are lower. For roads with less than 400 vehicles per day, see the AASHTO Guidelines
for Geometric Design of Very Low-Volume Local Roads (ADT <400).
Stopping sight distance is measured using an average eye height of 42 inches and an object
height of 24 inches (see Figure 5). On horizontal curves, stopping sight distance is measured
along the center of the inside lane, as shown in Figure 6.
Intersection sight distance
Intersection sight distance is often longer than stopping sight distance. A driver at an intersection
needs be able to see far enough to tell whether it is safe to proceed. In addition to traffic speed,
the intersection sight distance depends on the maneuver being made (through movement, left or
right turn) and the traffic control used at the intersection. Intersection sight distance is discussed
in detail in Chapter 6: Intersections, Driveways, and Railroad Crossings.
We cannot control the weather, but we can make allowances for its effects. Road characteristics
like good drainage, and skid-resistant road surfaces can reduce the number of poor-weather
accidents, while poor drainage or slippery pavement will increase them. The Roadway and
Roadside Drainage and Snow and Ice Control manuals published by the CLRP contain more
information on weather and roads.