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Is Speed Variance the Real Problem?

*N. J. Garber and R. Gadiraju. "Factors Affecting Speed
Variance and Its Influence on Accidents." Transportation Research Record,
1213 (1989), p. 64.*

*"A major influence on speed variance is the
difference between the design speed and the posted speed limit. It was
determined that speed variance will approach minimum values if the posted
speed limit is between 5 and 10 mph lower than the design speed. Outside
this range, speed variance increases... It was also found that drivers
tend to drive at increasing speeds as the roadway geometric characteristics
improve, regardless of the posted speed limit, and that accidents do not
necessarily increase with an increase in average speed but do increase
with an increase in speed variance."*
*C.M. Elmberg. Effects of Speed Zoning in Urban Areas.
M.S. Thesis. Purdue University, Layfayette, Indiana, May 1960.*

*"... a low posted speed limit on a highway
with good geometric conditions may result in a wide range of speeds on
the highway, which in turn will lead to an increase in accident rates."*

*N. J. Garber and R. Gadiraju. "Factors Affecting Speed
Variance and Its Influence on Accidents." Transportation Research Record,
1213 (1989), p. 69.*

*"... average speed depends on the design speed
and that speed variance depends upon average speed.... This model suggests
that minimum speed variance will occur when the difference between the
design speed and the posted speed limit is 10 mph."*

*Charles A. Lave. "Speeding, Coordination, and the 55
MPH Limit." The American Economic Review, 75.5 (1985 December), pp. 1159,
1161:*

*"I find that there is no statistically discernible
relationship between the fatality rate and average speed, though there
is a strong relationship to speed variance.... Variance kills, not speed."*

*"Simple physics indicates that the consequence
of a collision is a function of crash speed; and simple logic indicates
that the probability of collision is a function of the dispersion of speeds
on a given highway - more passing means more chances to collide."*