Weekday free-flowing speeds on the Solomon Hochoy Highway

Some authorities set speed limits to reflect the “reasonable and prudent” behaviour of the majority of motorists acting in an appropriate manner. This encourages drivers to obey the posted speed limit and travel at a reasonable speed.

It also targets limited enforcement resources at the occasional violator who disproportionately contributes to crash risk. According to the US Federal Highway Administration (FHWA), the concept of a rational speed limit involves a formal engineering review, during which drivers’ free-flowing speeds are observed. The assumption is that by reflecting actual driver speeds, most people will consider the speed limit appropriate. Such speed limits are desirable because they encourage public compliance, reduce speed differences among drivers, and offer a defensible enforcement tool.

The steps in the engineering approach to setting speed limits include planning, coordination, data collection and analysis, and finally, determination of the speed limits. A typical study of speed limits includes the following:

• Review of the road’s environment, features, and condition and traffic characteristics;

• Observation and measurement of vehicle speeds at one or more representative spots along the road in ideal weather and under free-flowing traffic conditions;

• Analysis of vehicle speeds to determine 85th percentile speed and other characteristics;

• Review of the road’s crash history; and,

• Review of any unusual conditions not readily apparent.

Most engineering approaches to speed limit setting are based on the 85th percentile speed — the speed at, or below which 85 percent of free-flowing traffic is traveling. The typical procedure is to set the speed limit at or near the 85th percentile speed of freeflow traffic. Adjustments to either increase or decrease of the speed limits may be made depending on infrastructure and traffic conditions.

FHWA (2012) Methods and Practices for Setting Speed Limits: An Informational Report, states that, “Setting a speed limit based on the 85th percentile speed was originally based on safety.

Specifically, re-search at the time had shown that travelling at or around one standard deviation above the mean operating speed (which is approximately the 85th percentile speed) yields the lowest crash risk for drivers.

Furthermore, crash risk increases rapidly for drivers travelling two standard deviations or more above or below the mean operating speed. Therefore, the 85th percentile speed separates accept-able speed behaviour from unsafe speed behaviour that disproportionately contributes to crash risk.

“The 85th percentile speed method is also attractive because it reflects the collective judgment of the vast majority of drivers as to a reasonable speed for given traffic and roadway conditions. This is aligned with the general policy sentiment that laws (ie, speed limits) should not make people acting reasonably into lawbreakers.

Setting a speed limit even 5 mph (8 km/h) below the 85th percentile speed can make almost half the drivers illegal; setting a speed limit 5 mph (8 km/h) above the 85th percentile speed will likely make few additional drivers legal.

Under the operating speed method of setting speed limits, the first approximation of the speed limit is to set the speed limit at the 85th percentile speed. The MUTCD [Manual on Uniform Traffic Control Devices] recommends that the speed limit be within 5 mph (8 km/h) of the 85th percentile speed of freeflowing traffic. The posted speed limit shall be in multiples of 5 mph, or 10 km/h for jurisdictions that employ metric.” During March this year, my students collected and analysed speed data during weekday daytime free-flowing periods at spots on the Solomon Hochoy Highway at the following segments:

1. Chaguanas Flyover and Carlsen Field Interchange;

2. Carlsen Field Interchange and Freeport Interchange;

3. Freeport Interchange and Couva Interchange;

4. Couva Interchange and Claxton Bay Interchange; and,

5. Gasparillo Interchange and Tarouba Interchange.

(We left out the segment between Claxton Bay Interchange and Gasparillo Interchange).

The 85th and 50th percentile values were as follows:

1. 74 and 58 kmph

2. 93 and 75 kmph

3. 75 and 60 kmph

4. 155 and 126 kmph

5. 134 and 106 kmph

The lower speeds in segments 1 and 3 might be because the Police are often seen in segment

1 at the walkway near Seereeram Brothers Ltd, and in segment

2 near the highway roti shop.

Where statutory limits do not fit specific road, traffic, or land uses conditions, road authorities have the power to establish speed zones to reflect the safe maximum reasonable speed. The following should be included for consideration:

• Residential zone speed limits;

• School zone speed limits;

• Road work zone speed limits;

• Truck speed limits; and,

• Minimum highway speed limits.

e-mail: info@ccost.org