The speed limit on South Santa Fe Drive is due for an increase from the unrealistic 45 mph to 55 mph between Iowa and Belleview avenues. Inside Lane photo.

By Kevin Flynn
Inside-Lane.com

The speed limit on South Santa Fe Drive’s expressway segment has long been under-posted at 45 mph. Most of the time outside rush hours, that seemed to be the slowest any driver went. Including the police.

But now, the Colorado Department of Transportation is expected to raise the limit to 55 mph, reflecting what the traffic already is safely doing.

The change would be in the 4½-mile segment between Iowa and Belleview avenues, passing through Denver, Englewood and Sheridan.

Drivers will benefit in at least two ways.

Many of them will no longer be caught in periodic ticket-writing sweeps of motorists who are driving the safe speed that the roadway and traffic allow.

Plus, they just might find it’s a safety improvement.

That’s right. Motorist safety tends to improve when traffic is moving along in a close range of speeds. People say speed kills, but more accurately, it’s differential in speed that kills. When posted speed limits are substantially less than the prevailing speed of traffic, the wider variation in speeds that result from some drivers observing the lower posted limit, mixing with those who are exceeding it, increases the chances of accidents.

Speed limit increase on South Santa Fe would be between Iowa and Belleview Avenues.

General practice says that speed limits should be set by measuring free-flowing traffic and determining the 85th percentile speed – the speed at or under which 85 percent of all drivers are traveling. Most of the vehicles tend to bunch around there. That number, rounded up to the closest multiple of 5, would be the speed limit.

Studies have shown that drivers tend to instinctively select the speed at which they feel safe and comfortable. Outliers who either intend to go faster or need to go slower are filtered out of the calculation by using the 85th percentile speed.

South Santa Fe has long been in need of change. Several years back, while commuting to work, I got in line behind Denver’s photo radar van when it pulled into traffic on Hampden Avenue at Sheridan after clicking automated ticket pictures of speeders there. I followed it off the exit at Santa Fe as it headed north toward downtown.

The photo radar van kept pulling away from me at 45, so I decided to keep pace with it. Just north of the Evans Avenue overpass – right where Denver sometimes still posts the speed van despite a state law that restricts where it can be used – the enforcement van topped out at 66 mph.

CDOT has actually found that a roadway’s average speed can be reduced when it raises the speed limit.

A speed limit that reflects what most traffic actually is doing prompts faster drivers to slow down because there is less need to pass slower traffic. That’s what happened when CDOT raised the limit on the Sixth Avenue Freeway in Lakewood in 2001 from 55 to 65 between Sheridan Boulevard and Interstate 70.

Data showed 85th percentile speeds dropped by up to 5 mph after the limit was raised. While up to nine percent of drivers had been going faster than 70 before the increase, no drivers during the survey were going that fast after the limit was raised.

CDOT has similarly raised the limits on Interstate 25 through the Denver Tech Center and on up to Evans Avenue, as well as on Interstate 225 between I-25 and Parker Road.

But the city of Boulder recently talked CDOT into lowering the speed limit on a mile and a half of U.S. 36 between Baseline Road and Foothills Parkway from 65 to 60. CDOT agreed to a three-year test period to see if it has an impact on safety.

According to CDOT, U.S. 36 carries 72,100 vehicles a day in that segment, and it has seen a significant increase in traffic volumes in the last few years. Over the last 10 years, CDOT said, there has been by a nine-percent increase in median barrier collisions.

That may actually have little to do with the speed limit increase, which took place almost 10 years ago as well, and more to do with the significant rise in total traffic. It would be difficult to attribute the cause of the increase in accidents to the speed limit without further analysis.

“Crash rates on this portion of U.S. 36 are not any different than other portions of US 36 or even similar types of highways, but we have seen an increase in median barrier collisions, which could be speed-related,” said CDOT Traffic Engineer Ina Zisman. “By lowering the speed limit for a three-year period, we can study the stretch and determine if speed is a factor and if the new speed limit has helped reduce median barrier collisions.”

To the extent that it fosters wider variations in motorist speeds, it may actually decrease safety.

One of Boulder’s concerns is highway noise in adjacent neighborhoods, and slower speeds produce less noise. But noise walls are usually the way to address that concern. Instituting a lower speed limit on a roadway on which prevailing speeds are higher is likely to foster widespread driver disregard of the lower limit and potentially decrease safety.