Speed limits based on safety, prudence, highway conditions

Questions occasionally surface about how ITD and local jurisdictions establish speed limits for routes under their control. Decisions are based on what is "safe and prudent" for driving conditions, ITD explains. The posted limit is a maximum speed when there are no obstacles or distractions and the pavement is dry. Those "clear" conditions are not always the norm.

"Speed-limit signs cannot replace common sense," said ITD's District 3Traffic Manager, Kevin Sablan. "They are intended to supplement - not substitute for - the driver's judgment."

Limits help determine a reasonable speed for particular road and weather conditions. Speed Limits are imposed to assist Idaho law enforcement. They encourage better traffic flow by reducing the variances in speed from one vehicle to the next, enhancing free-flowing movement.

Traffic limits that reflect the behavior of the majority of motorists are the most successful. Laws arbitrarily restricting the majority of drivers encourage wholesale violations, lack public support and generally fail to produce desirable changes in driving behavior.

In accordance with federal guidelines, ITD traffic engineers set the limit at the speed at which 85 percent of the traffic is driving to determine a safe and reasonable speed for a given road section. This reflects the safe speed as determined by a large majority of drivers. According to research, crash involvement is the lowest within that 85 percent.

Speed limits are also determined by a combination of engineering and traffic investigations.

The engineering investigation involves determining the design of the road and its immediate environment. Engineers analyze items such as lane width, pavement type and condition of the road. They also look at terrain, parking conditions, residential development along the road and the number, width and types of entrances and intersecting streets.

Reducing a speed limit does not necessarily result in safer driving conditions. Reducing the limit below the warranted speed can actually be hazardous and unsafe.

Studies have shown that merely reducing a speed limit has little effect on the speed at which motorists will travel. Enforcement of the speed limit by the law-enforcement community is essential. Furthermore, no published research findings have established any direct relationship between posted speed limits and crash frequency.

When determining speed limits, engineers attempt to set a realistic limit that the majority of drivers will obey and that can be reasonably enforced.

Published 10-28-2011