Highway Safety

 

David Weber, age 23, is a civil engineer in charge of safety improvements for District 7 (and eight-county area within a midwestern state). Near the end of the fiscal year, the district engineer informs David that delivery of a new snow plow has been delayed, and as a consequence the district has $50,000 in uncommitted funds. He asks David to suggest a safety project (or projects) that can be put under contract within the current fiscal year.

After a careful consideration of potential projects, David narrows his choice to two possible safety improvements. Site A is the intersection of Main and Oak Streets in the major city within the district. Site B is the intersection of Grape and Fir Roads in a rural area.

Pertinent data for the two intersections are as follows:

	Site A	Site B
Main road traffic (vehicles/day)	20,000	5,000
Minor road traffic (vehicles/day)	4,000	1,000
Fatalities per year (3 year average)	2	1
Injuries per year (3 year average)	6	2
PD* (3 year average)	40	12
Proposed Improvement	New signals	New signals
Improvement Cost	$50,000	$50,000

* PD refers to property damage only accidents.

A highway engineering textbook includes a table of average reductions in accidents resulting from the installation of the types of signal improvements David proposes. The tables are based on studies of intersections in urban and rural areas throughout the United States, over the past 20 years.

	Urban	Rural
% reduction in fatalities	50	50
% reduction in injuries	50	60
% reduction in PD	25	-25*

* Property damage only accidents are expected to increase because of the increase in rear-end accidents due to the stopping of high-speed traffic in rural areas.

David recognizes that these reduction factors represent averages from intersections with a wide range of physical characteristics (number of approach lanes, angle of intersection, etc.); in all climates; with various mixes of trucks and passenger vehicles; various approach speeds; various driving habits; and so on. However, he has no special data about Sites A and B that suggest relying on these tables is likely to misrepresent the circumstances at these sites.

Finally, here is some additional information that David knows about.

(1) In 1975, the National Safety Council and the National Highway Traffic Safety Administration both published dollar scales for comparing accident outcomes, as shown below:

	NSC	NHSTA
Fatality	$52,000	$235,000
Injury	$3,000	$11,200
PD	$440	$500