UT Dash Cam

Notes on the Speed Limit

Recently the speed limits for I-15, I-215, and I-80 in Salt Lake County have increased from 65mph to 70mph (with the exception of a few areas on I-80). This has caused quite the debate on the SL Tribune, Deseret News, and KSL sites.

Under many layers of emotional appeal, opponents of the idea seem to have a few major points:
1 – Decreased safety
2 – Increased emissions

I intend to show that both of these are relatively groundless, at least for the time being. I will also show a few other things that may be contributing to the change.  It should be noted beforehand that I’m in favor of the increase.

Read past the break for more details…

Initially, this mirrors the national debate when states raised the 55mph speed limit in 1987. Many people predicted both of the above results. They were wrong. Deaths declined. A study around this time determined that it was more dangerous to have different people going faster and slower (ie caused a few people following the speed limit, and many flouting it) than having people all drive faster together. This result has been replicated more recently, and is the cause of Google’s self-driving cars being programmed to deliberately exceed the speed limit.
Most speed-related accidents actually seem to be the result of driving too fast for the conditions, not just too fast in general. This finding has spurred development of variable speed limit signs; particularly for areas of frequent bad weather, like Parley’s Canyon in Utah. There, recently installed variable speed limit signs are deemed a great success.

Now, specific to the first point, will increasing the speed limit by 5mph in Salt Lake increase accidents directly attributable to excessive speed? (not merely speed excessive for the conditions)
The answer: Not likely.

Over the past few years, Utah has increased the rural Interstate speed limit (I-15, I-80, etc) from 75mph to 80mph. As with this current increase, there was an immediate storm of protest. Naysayers argued that fatalities would drastically rise, as previously-survivable wrecks gained impact damage from the higher velocity. A KSL article from a 2009 testing phase of the speed limit increase noted that there were fewer speeders and the average speed did not measurably increase. Three years later, after these speeds were deemed safe, much of the state Interstate system had a similar speed limit increase. UDOT data at the time showed that there had been a decline in accidents, due to “more vehicles traveling at the same speeds and less variation in the speed of surrounding vehicles.” A recent article increased the timespan of these observations to the past five years since the 2009 increase.

Returning to the present, UDOT data apparently shows that 85% of drivers are already driving at the new 70mph speed limit. Thus they don’t believe traffic will move faster. In addition, the UHP has pledged to more strictly enforce this new high speed limit.

In short, traffic is already moving at the new speed and the few cars that weren’t will now feel free to do so, thus traffic flow should end up more uniform. This should actually increase safety in the long run. Additionally, average speed shouldn’t increase noticeably (if prior trends hold true).
This being said, it is entirely possible that in ten years we will be driving that much faster, and the average speed will be much higher. However, for the foreseeable future and given previous experience, this is not likely to be the case.

The second point, regarding pollution, is much less certain.

It is a mathematical fact that cars generate more emissions as they go faster. Newer cars have mitigated this somewhat over older cars due to better design, but the fundamental point remains: every car has an optimal speed at which it maximizes the combination of distance covered vs fuel used. Thus it would seem that by traveling faster, we would see a proportionate increase in emissions (and also a lower air quality in general; however slight).

The problem with this is that (as noted above) the vast majority of drivers are already traveling at the new speed limit (if not higher). Increasing the speed limit has little detrimental effect on their efficiency (or subsequent emissions) – they’re already going at the new speed, thus their emissions will not increase. Additionally, there will be less acceleration and deceleration caused by dealing with backups caused by slower-moving cars – smoother traffic is more efficient.
Provided that the average speed doesn’t measurably increase (a supposition backed by previous experience with rural speed limit increase as noted above), emissions should also not measurably increase.

From my personal experience (obviously limited to one person), I’ve not noticed anything different in driving behavior. Many cars are still going in the 60-65mph range (especially around exits), even during low-traffic times of day.

Finally, what about other influences on the speed limit? The single most compelling point I have noticed that may have prompted this change is the average speed of the HOV lane. This may even have enough weight to completely overrule opposing conclusions to the two above points.
How does the HOV speed affect the general speed limit? It all comes down to funding.

It is apparently Federally-mandated that the average speed of the HOV lane must stay above 45mph. As of the end of November, the HOV average speed was only 45.5mph.
The penalty for not meeting this requirement is as follows: If the Administrator determines that a State has violated or failed to comply with the Federal laws or the regulations in this part with respect to a project, he may withhold payment to the State of Federal funds on account of such project, withhold approval of further projects in the State, and take such other action that he deems appropriate under the circumstances, until compliance or remedial action has been accomplished by the State to the satisfaction of the Administrator.

In other words, if the average speed is not high enough, Utah risks losing Federal funds for various transportation projects.

How does increasing the speed limit play into this?
Higher speed = Higher average speed
Increase the speed limit = Increase the average speed
Higher average speed = Continued money

Perhaps this has something to do with the hurry at which this increase was implemented.