Understanding what factors are contributing to accidents is the key to countermeasure selection.
The countermeasure should be targeted at a particular crash type or contributing factor. The goal
is to reduce the number and severity of accidents that occur.
There are no cure-alls for safety problems. No countermeasure works for all types of accident
patterns. Traffic signals, for example, do not always reduce the number of accidents. They
usually reduce the number of right-angle accidents. Rear-end accidents may increase or decrease
after a signal is installed. If there have not been many right-angle accidents at an intersection,
then an increase in rear-end accidents may be worse than the reduction in right-angle accidents.
NYSDOT and other agencies publish accident reduction factor lists that describe how much of a
decrease you can expect after using a given countermeasure. For example, adding arrow signs or
chevrons to a bad curve has resulted in 34 percent reductions in run-off-road accidents. Accident
reduction factors can be found at www.dot.state.ny.us/traffic/files/rftab95a.pdf
Typically, simple and inexpensive solutions are considered first. Realigning a curve can result
in a 79 percent reduction in run-off-road accidents, but it is very expensive. Curve and chevron
signs are usually tried first. If they do not work well enough, then consider more costly options.
This helps make sure the expensive options are saved for where they are really needed.