Police Chief Superintendent Alasdair Robinson said the dissident IRA bomb left Thursday night under a freeway overpass was properly constructed but abandoned before it could be detonated. He said it was likely that the attackers stopped short of their intended target because of a police road checkpoint.

The bomb — roughly double the size of a half-dozen car bombs that Irish Republican Army dissidents detonated last year in Northern Ireland — could have caused “huge devastation and loss of life,” if it had exploded in a town center, Robinson said. Last year’s bombs all caused little damage and no serious injuries.

The bomb alert closed the major road and rail line connecting Belfast and Dublin for all of Friday. It also showcased the public’s low regard for the threat posed by dissident IRA bombers, whose devices usually fail to explode properly.

After police shut down the main road south of the border town of Newry, impatient motorists unwilling to take a diversion moved the traffic cones and sandbagged “road closed” signs. BBC Northern Ireland footage showed dozens of cars driving directly past the van Friday morning before police reinforced the barriers.

Police are continuing to question three suspected IRA dissidents over last weekend’s killing of Ronan Kerr, a 25-year-old Catholic police recruit, who died when a booby-trap bomb detonated under his car in his driveway. He was the first member of Northern Ireland’s security forces to be killed by IRA dissidents in two years.

Police say several splinter groups opposed to the IRA’s 2005 decisions to disarm and renounce violence are trying to increase their violence in the run-up to a May 5 election of the Northern Ireland Assembly. The electoral campaign follows a surprisingly successful, stable four-year run for the British territory’s new Catholic-Protestant administration, whose members all face re-election.

Gerry Adams, leader of the IRA-linked party Sinn Fein, condemned the dissidents responsible for killing Kerr.

“The people of this island demand that you stop,” Adams wrote on his blog. “Your achievement has been to unite us all in opposition to your actions. It is time to end these futile attacks on the peace process, they will not succeed.” He told supporters of the dissidents: “Don’t be fooled into thinking that you are helping the IRA. The war is over. The IRA is gone. The IRA embraced, facilitated and supported the peace process. When a democratic and peaceful alternative to armed struggle was created the IRA left the stage.”