John Gooday, Scottish Road Works Commissioner, said he had urged councils to prosecute firms that were taking too long to carry out repairs to underground pipes and cables, but that few authorities had responded.
Last year only 76 notices had been issued under the powers of the New Roads and Street Works Act 1991, under which firms can be fined up to £1000 if they are not taking "reasonable" steps to complete works as quickly as possible.
This represents a 68% drop on 2009, when 242 notices were issued.
Mr Gooday said he had issued official guidance last October urging roads authorities to make use of the powers at their disposal to speed up works but that many were "still reluctant" to do so.
This is despite 19% of utility works taking longer than scheduled, representing more than 15,000 works across Scotland last year. A further 4% of repairs overran without notice being given.
The findings angered motoring groups, who said not enough was being done to ensure roads disruption is being kept to a minimum.
An AA spokesman said: "Utility firms have a privileged position in that they are able to dig up roads to lay utilities that make them big profits. It's entirely wrong if drivers are being held up because they are taking too long."
The number of delays is down on 2009, when 27% of roadworks undertaken by utility firms had their completion date extended and 5% overran without notice.
The figures are contained in the annual report of the Road Works Commissioner.
Mr Gooday said the low level of prosecutions against utility companies was due to procurators-fiscal being "generally unwilling" to pursue such offences and roads authorities worrying about time and cost involved.
John Franklin, RAC spokesman, said: "It's good there are fewer delays to roadworks (by utility companies) but with one in five taking longer than expected, you have to question the planning timescales they have in place."
However, Cosla, the organisation that represents Scotland's 32 local authorities, defended the low level of prosecutions, saying they did not always represent good value for money.
A spokesman said: "Delays in completion of roadworks by utility companies are only one part of the issue of roads maintenance.
"It is not always the case that legal action under Section 125 (3) Notices represents the best value for the public.
"Cosla understands the road works commissioner is undertaking work with the utilities and is due to report findings in the autumn. We will be looking to this report in further developing joint work with the utilities."