Guidelines for Spring Road Use Restrictions

by Doyt Y. Bolling, Director

taken from a presentation made at the 1999 City Streets & County Road School

from On the Move, Summer (June) Quarter 1999, Volume 12, Number 2

Many local agencies in Utah have their road and street networks significantly impacted by heavy vehicle axle loadings during the spring thaw and extreme wet weather conditions. Utah State law allows these local agencies to impose seasonal load restrictions to mitigate damage to their roads and streets. HB 72-7-408 Highway Authority specifically outlines the conditions under which these restrictions can be imposed.

72-7-408. Highway authority -- Restrictions on highway use -- Erection and maintenance of signs designating restrictions -- Penalty.

(1) (a) Subject to subsection (1) (b), a highway authority may by rule or ordinance prescribe procedures and criteria which prohibit the operation of any vehicle or impose restrictions on the weight of a vehicle upon any highway under its jurisdiction.

(b) A highway authority may impose restrictions under Subsection (1) (a) if an engineering inspection concludes that due to deterioration caused by climatic conditions a highway will be seriously damaged or destroyed unless certain vehicles are prohibited or vehicle weights are restricted.

(2) A highway authority may not prohibit the operation of any vehicle or restrict the weight of a vehicle under Subsection (1) for more than 90 days in any one calendar year.

(3) The highway authority imposing restrictions under this section shall erect signs citing the provisions of the rule or ordinance at each end of that portion of any highway affected. The restriction is effective only when the signs are erected and maintained.

(4) Any person who violates any restriction imposed under the authority of this section is guilty of a class B misdemeanor.

The key factors to consider in applying load restrictions include:

• pavement structure (structural number and layer thicknesses)
• type of subgrade
• surface deflections
• local experience--observed moisture and pavement distress

For example, local agencies should consider placing load restrictions if the following conditions exist:

• pavement surface is two inches or less and degree days are greater than 400 (modest depth of freezing)
• subgrades are fine-grained such as silts and clays
• pavement surface deflections during spring thaw are 40-45% higher than during the summer

Local experience will tell agencies whether other conditions exist to warrant placing load restrictions. For example:

• poor drainage from side ditches
• high ground water table or high winter precipitation
• pavement distress that occurs during early winter and spring

Studies show that load restriction amounts range from 20-60%. The average load restriction is approximately 44%. It should be noted that as load reduction percentages increase, pavement life increases. For example, a load reduction of 20% increases the pavement life by as much as 62%. A load reduction of 50% increases the pavement life by as much as 95%!

Local agencies should be judicious in determining the level of load restrictions to apply. Thin or generally weak pavement structures need high levels of load restrictions during Spring thaw. Unpaved roads on poor subgrade are more susceptible to damage from heavy vehicles. The general rules for load restrictions are as follows:

• minimum load restriction is 20%
• for paved roads, load restrictions greater than 60% are excessive

Determining when to apply load restrictions and when to remove them is based on air temperature data in terms of degree days of thawing. To determine the duration of load restrictions local agencies need to approximate the time required to achieve complete thawing.

For more information on how to determine when to apply load restrictions and how long they should be in place, please check out our website [http://www.utaht2.usu.edu/]. You are also welcome to call us at 1-800-822-8878 and request a copy of this presentation with complete information on placing Spring road use restrictions.